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<ul>
<li><a href="#chapter-4-various-types-and-mechanisms-of-degradation-reactions">CHAPTER 4 Various Types and Mechanisms of Degradation Reactions</a></li>
<li><a href="#第四章-各种类型的降解反应及其机理">第四章 各种类型的降解反应及其机理</a><ul>
<li><a href="#elimination">4.1 Elimination</a></li>
<li><a href="#消除反应">4.1 消除反应</a><ul>
<li><a href="#dehydration">4.1.1 Dehydration</a></li>
<li><a href="#失水反应">4.1.1 失水反应</a></li>
<li><a href="#dehydrohalogenation">4.1.2 Dehydrohalogenation</a></li>
<li><a href="#脱卤化氢反应">4.1.2 脱卤化氢反应</a></li>
<li><a href="#hofmann-elimination">4.1.3 Hofmann Elimination</a></li>
<li><a href="#hofmann-消除">4.1.3 Hofmann 消除</a></li>
<li><a href="#miscellaneous-eliminations">4.1.4 Miscellaneous Eliminations</a></li>
<li><a href="#其他消除反应">4.1.4 其他消除反应</a></li>
</ul></li>
<li><a href="#decarboxylation">4.2 Decarboxylation</a></li>
<li><a href="#脱羧反应">4.2 脱羧反应</a></li>
<li><a href="#nucleophilic-conjugate-addition-and-retro-nucleophilic-conjugate-addition">4.3 Nucleophilic Conjugate Addition and Retro-nucleophilic Conjugate Addition</a></li>
<li><a href="#亲核共轭加成及其逆反应">4.3 亲核共轭加成及其逆反应</a></li>
<li><a href="#aldol-condensation-and-retro-aldol">4.4 Aldol Condensation and Retro-aldol</a></li>
<li><a href="#羟醛缩合及其逆反应">4.4 羟醛缩合及其逆反应</a><ul>
<li><a href="#aldol-condensation">4.4.1 Aldol Condensation</a></li>
<li><a href="#羟醛缩合">4.4.1 羟醛缩合</a></li>
<li><a href="#retro-aldol-reaction">4.4.2 Retro-aldol Reaction</a></li>
<li><a href="#逆羟醛缩合">4.4.2 逆羟醛缩合</a></li>
</ul></li>
<li><a href="#isomerization-and-rearrangement">4.5 Isomerization and Rearrangement</a></li>
<li><a href="#异构化和重排">4.5 异构化和重排</a><ul>
<li><a href="#tautomerization">4.5.1 Tautomerization</a></li>
<li><a href="#互变异构">4.5.1 互变异构</a></li>
<li><a href="#racemization">4.5.2 Racemization</a></li>
<li><a href="#消旋化">4.5.2 消旋化</a></li>
<li><a href="#epimerization">4.5.3 Epimerization</a></li>
<li><a href="#差向异构化">4.5.3 差向异构化</a></li>
<li><a href="#no-acyl-migration">4.5.5 N,O-Acyl Migration</a></li>
<li><a href="#no-酰基迁移">4.5.5 N,O-酰基迁移</a></li>
<li><a href="#rearrangement-via-ring-expansion">4.5.6 Rearrangement via Ring Expansion</a></li>
<li><a href="#扩环重排">4.5.6 扩环重排</a></li>
<li><a href="#intramolecular-cannizzaro-rearrangement">4.5.7 Intramolecular Cannizzaro Rearrangement</a></li>
<li><a href="#分子内-cannizzaro-反应">4.5.7 分子内 Cannizzaro 反应</a></li>
</ul></li>
<li><a href="#cyclization">4.6 Cyclization</a></li>
<li><a href="#环化反应">4.6 环化反应</a><ul>
<li><a href="#formation-of-diketopiperazine-dkp">4.6.1 Formation of Diketopiperazine (DKP)</a></li>
<li><a href="#形成环缩二氨酸">4.6.1 形成环缩二氨酸</a></li>
<li><a href="#other-cyclization-reactions">4.6.2 Other Cyclization Reactions</a></li>
<li><a href="#其他环化反应">4.6.2 其他环化反应</a></li>
</ul></li>
<li><a href="#dimerizationoligomerization">4.7 Dimerization/Oligomerization</a></li>
<li><a href="#二聚齐聚">4.7 二聚/齐聚</a></li>
<li><a href="#miscellaneous-degradation-mechanisms">4.8 Miscellaneous Degradation Mechanisms</a></li>
<li><a href="#其他降解机理">4.8 其他降解机理</a><ul>
<li><a href="#diels-alder-reaction">4.8.1 Diels-Alder Reaction</a></li>
<li><a href="#diels-alder-反应">4.8.1 Diels-Alder 反应</a></li>
<li><a href="#degradation-via-reduction-or-disproportionate">4.8.2 Degradation via Reduction or Disproportionate</a></li>
<li><a href="#还原或歧化反应造成的降解">4.8.2 还原或歧化反应造成的降解</a></li>
</ul></li>
<li><a href="#references">References</a></li>
</ul></li>
</ul>
</div>
<table><tr><td width="59%" height="0"></td><td></td></tr>

<tr><td>

<h1 id="chapter-4-various-types-and-mechanisms-of-degradation-reactions"><a href="#chapter-4-various-types-and-mechanisms-of-degradation-reactions">CHAPTER 4 Various Types and Mechanisms of Degradation Reactions</a></h1>
</td><td>

<h1 id="第四章-各种类型的降解反应及其机理"><a href="#第四章-各种类型的降解反应及其机理">第四章 各种类型的降解反应及其机理</a></h1>
</td></tr>
<tr><td>

<h2 id="elimination"><a href="#elimination">4.1 Elimination</a></h2>
</td><td>

<h2 id="消除反应"><a href="#消除反应">4.1 消除反应</a></h2>
</td></tr>
<tr><td>

<p>An elimination reaction is one in which two substituents are removed or eliminated from the parent molecule. The elimination can proceed either via a one- or two-step mechanism, known as E2 and E1 elimination, respectively (Scheme 4.1). The numbers 1 and 2 here refer to the orders, rather than the steps, of the elimination reactions. In the E1 elimination, an intermediate is formed by the elimination of the first substituent, which is then followed by the elimination of the second substituent. The first step of the E1 elimination is usually the rate-liming step and hence the reaction is of first order relative to the elimination substrate. In the E2 elimination, a concerted mechanism is operative; frequently, a base is required to effect the concerted elimination of the two substituents. Therefore, the reaction is second order owing to the involvement of the two reactants.</p>
</td><td>

<p>消除反应中，将有两个取代基从反应物上离去。消除反应的两种机理——单步反应和两步反应，分别名为 E2 和 E1 消除 (Scheme 4.1)。此处的数字代表反应级数，而非反应所需几步。E1 消除时，先离去一个基团生成反应中间体，然后再离去另一个基团。第一个基团的离去往往是速率控制步骤，故此反应是一级反应，反应速率只与底物相关。E2 消除时，将发生协同机理。一般需要碱参与反应使得两个基团的协同离去。因此，E2 消除是双分子反应，其反应级数为 2。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.1.png" alt="Scheme 4.1  " /><p class="caption"><span class="pic-ref">Scheme 4.1</span>  </p>
</div>
</td></tr>
<tr><td>

<p>In both types of the elimination reaction, the double bond equivalency (DBE) of the resulting degradation product increases by one, that is, the elimination causes the formation of a double bond (from a single bond), a triple bond (from a double bond), or a ring. The most pharmaceutically relevant drug degradation via elimination is probably dehydration. Other elimination reactions that have been observed in pharmaceutical products include dehydrohalogenation (elimination of HX, where X = halogen such as Cl) and the Hofmann elimination. Decarboxylation is a degradation reaction that is closely related to, but is not elimination. They will be discussed in the following sub-sections, respectively.</p>
</td><td>

<p>无论哪种消除机理，反应产物的不饱和度(the double bond equivalency, DBE)都会增加 1，即消除反应可生成双键（原本是单键）、三键（原本是双键）或环。药物降解中涉及到的最重要的消除反应是失水反应。在制剂产品中曾观测到的其他消除反应还包括脱卤化氢反应(离去 HX，此处 X = 卤素，例如 Cl)和 Hofmann 消除。脱羧反应虽然也是联系密切的降解反应，但并不属于消除反应。下几节中将分别介绍这些反应。</p>
</td></tr>
<tr><td>

<h3 id="dehydration"><a href="#dehydration">4.1.1 Dehydration</a></h3>
</td><td>

<h3 id="失水反应"><a href="#失水反应">4.1.1 失水反应</a></h3>
</td></tr>
<tr><td>

<p>ehydration is probably the most commonly seen degradation type due to an elimination reaction. Several corticosteroids containing the 1,3-dihydroxyacetone side chain on the D-ring, such as dexamethasone, betamethasone, prednisolone, and Cortisol undergo a dehydration reaction known as the Mattox rearrangement (which is actually not a rearrangement), in particular under acidic conditions.<span class="cite-ref"><sup>[1,2]</sup></span> During the Mattox process as shown in Scheme 4.2, the side chains of the corticosteroids are presumed to enolize prior to the dehydration, which results in the formation of two regio-isomeric enol aldehydes as the degradants.</p>
</td><td>

<p>失水反应是药物降解中最常见的消除反应。多种皮质类固醇类药物，诸如 dexamethasone、betamethasone、prednisolone 和 Cortisol，它们的 D 环侧链上都有 1,3-二羟基丙酮结构，可发生失水反应，且在酸性条件下尤其显著。此反应名为 Mattox 重排（但严格来讲，此反应根本就不是重排反应）。<span class="cite-ref"><sup>[1,2]</sup></span> Mattox 反应的细节见 Scheme 4.2：皮质类固醇分子的侧链互变为烯醇式，然后发生失水反应生成一对异构体——顺反异构的醛基取代的烯醇。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.2.png" alt="Scheme 4.2  " /><p class="caption"><span class="pic-ref">Scheme 4.2</span>  </p>
</div>
</td></tr>
<tr><td>

<p>In the case of betamethasone, a competing dehydration process, in which the 17-hydroxyl group is eliminated along with the neighboring 16-hydrogen, producing 16Δ-betamethasone, was also observed.<span class="cite-ref"><sup>[3]</sup></span> It appears that the nature of the counter ion in the acid and the solvent used in the forced degradation has an impact on the distribution of the dehydration products: when stressed with HCl in dioxane, an appreciable amount of 16Δ-betamethasone was formed, while stressing with sulfuric acid in a mixture of acetonitrile and water produced no detectable level of the latter degradant<span class="cite-ref"><sup>[4]</sup></span> (Scheme 4.3).</p>
</td><td>

<p>Betamethasone 还能发生另一种脱水反应，17位的羟基与 16位的氢发生消除，生成 16Δ-betamethasone。<span class="cite-ref"><sup>[3]</sup></span> 而且强制降解中使用不同的酸或溶剂都似乎可以改变脱水产物的最终比例。当使用 HCl/二氧六环时，生成了一定比例的 16Δ-betamethasone；换用硫酸/乙腈、水混合溶液后，则没能检测出此降解杂质。<span class="cite-ref"><sup>[4]</sup></span> 详细反应见 Scheme 4.3。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.3.png" alt="Scheme 4.3  " /><p class="caption"><span class="pic-ref">Scheme 4.3</span>  </p>
</div>
</td></tr>
<tr><td>

<p>The 17,21-di-esters of betamethasone, betamethasone 9,11-epoxide, and dexamethasone also undergo elimination, analogous to the Mattox rearrangement, in which the equivalent of a molecule of carboxylic anhydride is removed.<span class="cite-ref"><sup>[4,5]</sup></span> This variation of the Mattox degradation process was shown to be catalyzed by a base or nucleophile (Scheme 4.4). Both the original Mattox process <span class="cite-ref"><sup>[1,2]</sup></span> and its variation with the di-esters<span class="cite-ref"><sup>[5]</sup></span> are likely to be concerted processes, that is, both should proceed via the E2 elimination mechanism.</p>
</td><td>

<p>Betamethasone 的 17,21-二酯、9,11-环氧化物和 dexamethasone 同样能发生类似于 Mattox 重排的消除反应，此时的离去基团是酸酐的等价物。<span class="cite-ref"><sup>[4,5]</sup></span> 此种 Mattox 重排的变体反应需要碱或亲核试剂的催化(Scheme 4.4)。此反应与经典的 Mattox 重排似乎都是协同进行的，即 E2 消除机理。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.4.png" alt="Scheme 4.4  " /><p class="caption"><span class="pic-ref">Scheme 4.4</span>  </p>
</div>
</td></tr>
<tr><td>

<p>In a number of products formulated with betamethasone dipropionate, betamethasone enol aldehyde has been observed to be a degradant, especially in liquid formulations.<span class="cite-ref"><sup>[6-8]</sup></span></p>
</td><td>

<p>在 betamethasone 的某些制剂产品中发现了降解产物 betamethasone enol aldehyde，且液体剂型中尤其明显。<span class="cite-ref"><sup>[6-8]</sup></span></p>
</td></tr>
<tr><td>

<p>For a drug like pridinol which contains a diphenylalkylcarbinol moiety, it would be expected that dehydration is at least a potential degradation pathway. Indeed, when a sample of pridinol was stressed with 0.1N HCl at room temperature for 6 days, Bianchini et al. found that ~ 0.3% of the dehydrated product was formed.<span class="cite-ref"><sup>[9]</sup></span> These workers also determined that the dehydration reaction is first order with an activation energy of 25.5 kcal mol<sup>-1</sup>. This elimination reaction probably takes place via an E1 mechanism, owing to the stabilization of the initially formed carbocation by the two phenyl groups (Scheme 4.5).</p>
</td><td>

<p>像 pridinol 这样含有二苯烷基甲醇结构的药物分子，是可能发生发生失水反应的（至少从理论上看来是这样）。实际上，Bianchini 等人以 0.1N HCl 溶液强制降解 pridinol，室温放置 6 天，约有 ~ 0.3% 的脱水产物生成。<span class="cite-ref"><sup>[9]</sup></span> 他们还确定了此失水反应是一级反应，活化能为 25.5 kcal mol<sup>-1</sup>。此消除反应可能是经历了 E1 机理，毕竟在两个苯环的共轭下，碳正离子可稳定存在(Scheme 4.5)。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.5.png" alt="Scheme 4.5  " /><p class="caption"><span class="pic-ref">Scheme 4.5</span>  </p>
</div>
</td></tr>
<tr><td>

<p>For drug molecules that contain multiple hydroxyl groups, dehydration by intramolecular condensation between two hydroxyl groups can occur. Erythromycin, an important member in the macrolide antibiotics family, possesses a number of hydroxyl groups. Erythromycin A, which is the major component of erythromycin, has long been known to degrade in aqueous solution, particularly in acidic solutions. According to the studies performed by Atkins et al.<span class="cite-ref"><sup>[10]</sup></span> and Cachet et al.,<span class="cite-ref"><sup>[11]</sup></span> once dissolved in acidic to neutral solutions, erythromycin A quickly converted to two dehydrated species, erythromycin A enol ether and anhydroerythromycin A. Cachet et al. proposed a mechanism<span class="cite-ref"><sup>[11]</sup></span> that differs from the original mechanism proposed by Atkins et al.<span class="cite-ref"><sup>[10]</sup></span> (Scheme 4.6).</p>
</td><td>

<p>含有多个羟基的药物分子的两个羟基之间可能会发生分子脱水缩合水反应。重要的大环内酯类药物 Erythromycin 就含有多个羟基。erythromycin A 是其主要组分，已知此物在水溶液中容易降解，酸性 pH 尤其明显。Atkins 等人的研究<span class="cite-ref"><sup>[11]</sup></span>认为，erythromycin A 溶解于酸性至中性的溶液中后将迅速生成两个脱水产物：erythromycin A enol ether 和 anhydroerythromycin A。Cachet 等人<span class="cite-ref"><sup>[11]</sup></span>则提出了另一种降解机理(Scheme 4.6)。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.6.png" alt="Scheme 4.6  " /><p class="caption"><span class="pic-ref">Scheme 4.6</span>  </p>
</div>
</td></tr>
<tr><td>

<p>In the new mechanism postulated by Cachet et al., erythromycin A directly degrades to the dehydrated degradant, anhydroerythromycin A, while the original mechanism proposed by Atkins et al. suggested a consecutive degradation pathway. The new mechanism was supported by the observation that erythromycin A enol ether quickly gave rise to erythromycin A in aqueous solution. In addition, the kinetic model derived from the new mechanism fits the experimental data better.<span class="cite-ref"><sup>[11]</sup></span> In the above studies conducted by the two research groups, the detailed configuration and stereochemistry of the degradants and related compounds was not examined. Since the mid-1990s, efforts have been directed toward resolving this deficiency; Alam et al. confirmed the structure and stereochemistry of erythromycin A enol ether using 2D nuclear magnetic resonance (NMR),<span class="cite-ref"><sup>[12]</sup></span> while Hassanzadeh et al. established the stereochemistry of anhydroerythromycin A using 2D NMR, X-ray crystallography, and computer modeling.<span class="cite-ref"><sup>[13]</sup></span> Based on all the key results reported in the literature,<span class="cite-ref"><sup>[10-13]</sup></span> the overall degradation behavior of erythromycin A can be summarized in Scheme 4.7.</p>
</td><td>

<p>Cachet 等人提出的机理中，erythromycin A 直接降解产生脱水产物 anhydroerythromycin A；Atkins 等人则认为此物是由 erythromycin A enol ether 进一步降解而成。但有人发现在水溶液中，Erythromycin A enol ether 可迅速转化成为Erythromycin A。且Cachet 等人的机理所推倒出的动力学模型与实验数据拟合得更好。<span class="cite-ref"><sup>[11]</sup></span> 两个小组的研究中，都没能确定降解产物的构型和立体化学，且未检测有关物质情况。直到 90 年代中期才有人试图补全这一空白。Alam 等人利用 2D NMR 确定了 erythromycin A enol ether 的构型和立体化学。<span class="cite-ref"><sup>[12]</sup></span> 同时，Hassanzadeh 等人综合运用 2D NMR、X 射线晶体学和计算机模拟成功确定了 anhydroerythromycin A 的立体化学。<span class="cite-ref"><sup>[13]</sup></span> 综合这些人的关键研究成果<span class="cite-ref"><sup>[13]</sup></span>，可总结出 erythromycin A 降解的全貌，见 Scheme 4.7。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.7.png" alt="Scheme 4.7  " /><p class="caption"><span class="pic-ref">Scheme 4.7</span>  </p>
</div>
</td></tr>
<tr><td>

<p>In either of the two pathways illustrated in Scheme 4.7, it is apparent that a hemiacetal (6,9- or 9,12-hemiacetal), formed when either 6-hydroxyl or 12-hydroxyl attack the 9-keto position, should be the transient intermediate respectively. It should be noted that erythromycin A enol ether was sometimes incorrectly referred to as its intermediate, erythromycin A 6,9-hemiacetal. According to a study on solution stability of erythromycin A by Barber et al.,<span class="cite-ref"><sup>[14]</sup></span> the ratio of the native 9-keto form of erythromycin A versus the therapeutically inactive enol ether form is 5:2 in neutral solution at ambient temperature.</p>
</td><td>

<p>Scheme 4.7 中的两条降解途径，都是以某个半缩酮（6,9- 或 9,12-hemiacetal，分别由 6 位或 12 位羟基进攻 9 位羰基而成）为关键中间体。值得注意的是，时常会有人误认为 erythromycin A enol ether 会转化生成 erythromycin A 6,9-hemiacetal。参考 Barber 等人所做的溶液稳定性研究，室温下，中性溶液中，erythromycin A 与其无生物活性的烯醇酯形式的比例为 5:2。</p>
</td></tr>
<tr><td>

<p>Azithromycin, one of the best selling antibiotic drugs, was made by structure modification of erythromycin through the replacement of the 9-keto functionality by a methyl-substituted nitrogen moiety. Since the 9-keto functionality, which is essential for the formation of the two major degradants shown in Scheme 4.7, no longer exits, the stability of azithromycin in acidic solution is markedly improved: its main degradation pathway is the much slower hydrolysis of the ether linkage to the cladinose sugar substituent at the 3-position of the macrolide ring.<span class="cite-ref"><sup>[15]</sup></span></p>
</td><td>

<p>畅销药 azithromycin 是对 erythromycin 进行结构改造得到的，9 位羰基被换成了甲基取代的氮原子。而 9 位的羰基是 Scheme 4.7 中所示降解反应的关键，除去此基团后，药物分子在酸溶液中的稳定性显著提高：其主要降解途径变成了大环内酯的酯键，且水解非常缓慢。</p>
</td></tr>
<tr><td>

<p>Another frequently observed dehydration reaction is the formation of an intra-molecular amide or ester linkage between an amino or hydroxyl group and a carboxyl group, giving rise to a new ring in the degradant formed. A number of drugs based on dipeptides or dipeptide analogs tend to undergo this kind of dehydration yielding a type of degradants collectively known as diketopiperazines (DKPs). Since all the degradants are cyclic compounds, this degradation will be discussed as part of Section 4.6.</p>
</td><td>

<p>另一种常见的失水反应是分子内成酯或成酰胺。当分子中同时存在羧基和羟基（或氨基）时，可发生分子内反应生成内酯（或内酰胺）。一些二肽药物（或二肽相似物）就可以发生这种反应生成所谓的环缩二氨酸(diketopiperazines, DKPs)。鉴于此种降解发生了成环反应，故将于 小节 4.6 详细讨论。</p>
</td></tr>
<tr><td>

<h3 id="dehydrohalogenation"><a href="#dehydrohalogenation">4.1.2 Dehydrohalogenation</a></h3>
</td><td>

<h3 id="脱卤化氢反应"><a href="#脱卤化氢反应">4.1.2 脱卤化氢反应</a></h3>
</td></tr>
<tr><td>

<p>Mometasone furoate, a corticosteroid pro-drug widely used for a variety of anti-inflammation indications, possesses a 9α-chloro, 11β-hydroxyl moiety that is susceptible to dehydrochlorination. Two research groups studied the stability of mometasone furoate in simulated lung fluid, an aqueous solution made up of various salts and buffers with pH adjusted to 7.4.<span class="cite-ref"><sup>[16,17]</sup></span> Both groups found that mometasone furoate underwent dehydrochlorination rather quickly at 37 °C to yield mometasone furoate 9,11-epoxide; the half-life of this degradation was determined to be 1.3 hours by Sahasranaman et al.<span class="cite-ref"><sup>[11]</sup></span> The rate of dehydrochlorination increased as the pH of the solution was raised. The relatively facile elimination and its pH dependence may be attributed to the following factors: first, the two reacting groups, 11β-hydroxyl and 9α-chloro groups are already in the trans orientation that would be favored by an E2 elimination mechanism. Second, chloride is a reasonably good leaving group. Third, under higher pH, the nucleophilicity of the 11-hydroxyl group is enhanced.</p>
</td><td>

<p>广泛使用的镇静剂 Mometasone furoate 是一个皮质类固醇类的前药。其结构中的 9α-氯 和 11β-羟基 容易发生脱卤化氢反应。两个研究小组研究了它在模拟肺液中的稳定性，模拟肺液为含有多种盐，pH 7.4 的缓冲溶液。<span class="cite-ref"><sup>[16,17]</sup></span> 他们都发现 Mometasone furoate 在 37 °C是将很快地发生脱卤化氢反应生成 mometasone furoate 9,11-epoxide；Sahasranaman 测得此降解反应的半衰期为 1.3 小时。<span class="cite-ref"><sup>[11]</sup></span> 当 pH 升高时，反应速率随之提高。此物容易发生脱卤化氢反应且受 pH 影响可归因为：1) 两个反应基团，9α-氯 和 11β-羟基 处于反式，有利于发生 E2 消除。2) 氯离子是良好的离去基团。3) 提高 pH，11-羟基 的亲核性增强。</p>
</td></tr>
<tr><td>

<p>The latter authors also found that the 9,11-epoxide degradant underwent a further condensation reaction between the 20-methylene and the carbonyl group of the furoate moiety to give a dehydrated degradant.<span class="cite-ref"><sup>[11]</sup></span> The kinetic model fitting consistent with this overall sequential degradation pathway is illustrated in Scheme 4.8.</p>
</td><td>

<p>Sahasranaman 还发现降解产物 9,11-epoxide 的 20位亚甲基可与糠酸的羰基发生缩合反应生成新的脱水产物。<span class="cite-ref"><sup>[11]</sup></span> 此连续降解途径的动力学模型见 Scheme 4.8。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.8.png" alt="Scheme 4.8  " /><p class="caption"><span class="pic-ref">Scheme 4.8</span>  </p>
</div>
</td></tr>
<tr><td>

<p>In a stability study performed by Teng et al.,<span class="cite-ref"><sup>[16]</sup></span> a small amount of dehydrated mometasone furoate was observed; hence, the alternate pathway (dotted arrows) could not be ruled out as a minor contributor to the final degradant. The major sequential pathway seems to suggest that the conformation change in the steroid core structure induced by the formation of the 9,11-epoxide bond renders the subsequent condensation much easier.</p>
</td><td>

<p>Teng 等人通过稳定性研究发现，有少量脱水产物 Dehydrated mometasone furoate 生成。故此，另一条降解途径(虚线箭头所示)也可能对最终降解产物的生成有所贡献。前一种降解途径占主要，好像是甾体母核发生环氧化会使得下一步的缩合更容易进行。</p>
</td></tr>
<tr><td>

<p>A few other corticosteroids including mometasone (the precursor of its pro-drug, mometasone furoate), beclomethasone, and clocortolone, also possess the 9α-chloro,11β-hydroxyl moiety and hence could be susceptible to the same dehydrochlorination degradation,<span class="cite-ref"><sup>[5]</sup></span> especially in liquid formulations.<span class="cite-ref"><sup>[18]</sup></span> On the other hand, corticosteroids containing the 9α-fluoro,11β-hydroxyl moiety, such as betamethasone, dexamethasone, triamcinolone, and desoximetasone (Figure 4.1), would be less likely to undergo the corresponding dehydrofluorination reaction because fluoride is a poorer leaving group compared to chloride.</p>
</td><td>

<p>少数皮质类固醇药物（比如 mometasone 以及其前药 mometasone furoate、beclomethasone 和 clocortolone），分子中的 9α-氯 和 11β-羟基 容易发生类似的脱卤化氢反应<span class="cite-ref"><sup>[5]</sup></span>，液体制剂尤其明显<span class="cite-ref"><sup>[18]</sup></span>。另一方面，含有 9α-氟 和 11β-羟基 的皮质类固醇药物分子（比如 betamethasone、dexamethasone、triamcinolone 和 desoximetasone，见 Figure 4.1），发生相应脱卤化氢反应要略难一些，这是因为氟比氯更难离去。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/F4.1.png" alt="Figure 4.1   Betamethasone、dexamethasone、triamcinolone 和 desoximetasone 的结构式。Betamethasone, dexamethasone, triamcinolone, and desoximetasone." /><p class="caption"><span class="pic-ref">Figure 4.1</span>   Betamethasone、dexamethasone、triamcinolone 和 desoximetasone 的结构式。<br />Betamethasone, dexamethasone, triamcinolone, and desoximetasone.</p>
</div>
</td></tr>
<tr><td>

<h3 id="hofmann-elimination"><a href="#hofmann-elimination">4.1.3 Hofmann Elimination</a></h3>
</td><td>

<h3 id="hofmann-消除"><a href="#hofmann-消除">4.1.3 Hofmann 消除</a></h3>
</td></tr>
<tr><td>

<p>The Hofmann elimination, also known as Hofmann degradation or exhaustive methylation, is a reaction in which an amine is converted to the corresponding quaternary salt intermediate by methyl iodide and then pyrolyzed to yield an olefin through elimination of the amine moiety. In drug degradation chemistry, the relevance of the Hoffmann elimination is limited to those drug substances that already have a quaternary salt functionality. For example, widely used clinically skeletal muscle relaxants, atracurium and its 1R-cis, 1R'-cis isomer, cisatracurium contain two symmetric quaternary ammonium salt units which are linked together by a diester spacer. This spacer was purposely designed to be biodegradable in vivo, so that the drug has a shorter duration of action compared to the metabolically stable precursor drug, laudexium, upon which atracurium was developed.<span class="cite-ref"><sup>[19]</sup></span> Lability towards in vivo degradation is achieved through Hofmann elimination of the quaternary salt moieties. Unfortunately, this degradation pathway also occurs in aqueous buffers as shown in Scheme 4.9.</p>
</td><td>

<p>Hofmann 消除反应，又称 Hofmann 降解或彻底甲基化反应。此反应使用碘甲烷将胺转化为相应的季铵盐，然后热解消除生成烯烃。在药物降解中，涉及 Hofmann 消除的只有那些分子中含有季铵盐结构的分子。比如临床上广泛使用的骨骼肌松弛剂 atracurium 及其 1R-cis、1R'-cis 异构体。Cisatracurium 分子是对称的，两个结构相同季铵盐以酯键连接在正戊烷两端。相比于不易代谢 laudexium，atracurium 被故意设计成容易在体内降解，从而使其作用时间更短。<span class="cite-ref"><sup>[19]</sup></span> 在体内容易降解就是因为季铵盐结构可发生 Hofmann 降解。不幸的是，在缓冲溶液中也会发生此种降解反应，见 Scheme 4.9。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.9.png" alt="Scheme 4.9  " /><p class="caption"><span class="pic-ref">Scheme 4.9</span>  </p>
</div>
</td></tr>
<tr><td>

<p>The rate of the atracurium degradation was found to increase as the pH became higher, which is consistent with the above mechanism where deprotonation of the activated methylene moiety a to the carbonyl group triggers the Hofmann elimination.<span class="cite-ref"><sup>[20]</sup></span></p>
</td><td>

<p>当 pH 升高，降解速率变大。这符合上述机理，活性亚甲基脱质子化进而发生 Hofmann 消除。<span class="cite-ref"><sup>[20]</sup></span></p>
</td></tr>
<tr><td>

<p>Another example of drug degradation by Hofmann elimination can be found in the case of a drug candidate in the family of carbapenem. This drug candidate contains a side chain that is releasable upon the cleavage of the β-lactam linkage, in order to minimize the potential immunogenic side effect of the intact drug.<span class="cite-ref"><sup>[21]</sup></span> The side chain possesses a twin quaternary ammonium salt moiety, which is susceptible to Hofmann elimination and related degradation in both the solid and solution states (Scheme 4.10).<span class="cite-ref"><sup>[22,23]</sup></span></p>
</td><td>

<p>另一个例子是某碳青霉素烯类备选药物。此药物分子的侧链可与母核断裂，从而尽量减少潜在的免疫原副作用。<span class="cite-ref"><sup>[21]</sup></span> 此侧链含有两个季铵盐，无论是在固体或溶液状态都容易发生 Hofmann 消除而发生降解(Scheme 4.10)。<span class="cite-ref"><sup>[22,23]</sup></span></p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.10.png" alt="Scheme 4.10  " /><p class="caption"><span class="pic-ref">Scheme 4.10</span>  </p>
</div>
</td></tr>
<tr><td>

<p>As discussed above, one of the key factors triggering the Hofmann elimination is the availability of an &quot;activated&quot; methylene moiety β to the quaternary salt, which is also true in this case. The second key factor is the nature of the leaving group: a tertiary amine can be a reasonably good leaving group. In pathway b of Scheme 4.10, the Hofmann elimination is not possible due to the lack of a β-methylene moiety. However, attack on the α-methylene functionality by a nucleophile could lead to the removal of the acetamide moiety. The latter pathway (b) was observed in a stress study of a chloride salt of the drug (X = Cl), while it was absent in a study of a benzenesulfonate salt.<span class="cite-ref"><sup>[22]</sup></span> These results suggest that the nucleophile in pathway b could be the chloride since it is a stronger nucleophile than the benzenesulfonate.</p>
</td><td>

<p>如前所述，触发 Hofmann 消除的关键是季铵盐的 β 位存在活性亚甲基，此例亦然。其次是离去基团的性质：叔胺是一个良好的离去基团。Scheme 4.10 途径 b 中，由于缺少 β-氢，不会发生 Hofmann 消除。然而亲核试剂进攻 α亚甲基同样可发生分子断裂。此药物的盐酸盐在强制降解中观察到了降解途径 b （X = Cl）；但其苯磺酸盐却没有发生此种降解。<span class="cite-ref"><sup>[22]</sup></span> 这说明途径 b 中，氯离子可能作为亲核试剂。毕竟其亲核性比苯磺酸根强。</p>
</td></tr>
<tr><td>

<h3 id="miscellaneous-eliminations"><a href="#miscellaneous-eliminations">4.1.4 Miscellaneous Eliminations</a></h3>
</td><td>

<h3 id="其他消除反应"><a href="#其他消除反应">4.1.4 其他消除反应</a></h3>
</td></tr>
<tr><td>

<p>For keto-compounds containing a β-hetero atom such as oxygen, nitrogen, and sulfur, elimination can occur via the mechanism of retro-nucleophilic conjugate addition, sometimes also referred to as retro-Michael addition. This type of drug degradation will be discussed in Section 4.3.</p>
</td><td>

<p>羰基化合物若含有 β 杂原子（比如氧、氮、硫），可发生消除反应。其机理为逆亲核共轭加成，有时又称逆 Michael 反应。此种降解反应将在 小节 4.3 讨论。</p>
</td></tr>
<tr><td>

<h2 id="decarboxylation"><a href="#decarboxylation">4.2 Decarboxylation</a></h2>
</td><td>

<h2 id="脱羧反应"><a href="#脱羧反应">4.2 脱羧反应</a></h2>
</td></tr>
<tr><td>

<p>In general, carboxyl-containing compounds, except for those that contain an &quot;activator&quot; at the position β to the carboxyl group, do not undergo decarboxylation easily unless they are treated under fairly harsh conditions. In the transition state of the decarboxylation process, a negative charge develops at the position α to the carboxyl group, while a positive charge develops at the carboxyl hydrogen. In order for the decarboxylation to proceed, the negative charge needs to be neutralized or released by the &quot;activator&quot; at the β position, while the positive charge is captured so that the transition state can be stabilized. This explains why a carboxyl-containing compound that has a β-double bond is quite susceptible to decarboxylation, because the double bond fosters the formation of a stabilized transition state, from which the negative charge can reunite with the carboxyl proton according to the concerted mechanism in Scheme 4.11.</p>
</td><td>

<p>一般来说，含有羧基的化合物若非 β 位含有活化基团，基本不会轻易发生脱羧反应。一般的羧酸只有在及其苛刻的条件下才会发生脱羧反应。脱羧反应一般要经历一个过渡态，羧基 α 位带负电荷，羧基的氢带正电荷。此正电荷若被 β 位的活化基团中和或转移，过渡态则更稳定，脱羧也更容易进行。这就解释了 β 位为双键的羧基化合物容易发生脱羧的原因，双键稳定了六元环状过渡态，经协同机理发生脱羧反应(Scheme 4.11)。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.11.png" alt="Scheme 4.11  " /><p class="caption"><span class="pic-ref">Scheme 4.11</span>  </p>
</div>
</td></tr>
<tr><td>

<p>A great number of non-steroidal anti-inflammatory drugs (NSAIDs) contain a carboxyl group. 4-Aminosalicylic acid, one of the NSAIDs used for treating tuberculosis and inflammatory bowel diseases, is known to decarboxylate upon heating to produce CO<sub>2</sub> and 3-aminophenol.<span class="cite-ref"><sup>[24]</sup></span> The drug can tautomerize into the β-keto form, which is more susceptible to decarboxylation by the concerted mechanism discussed above. Scheme 4.12 shows tautomerization of the drug followed by decarboxylation.</p>
</td><td>

<p>许多非甾体类抗炎药( non-steroidal anti-inflammatory drugs, NSAID)都含有羧基。比如用于治疗结核病和炎症性肠疾病非甾体类抗炎药 4-Aminosalicylic acid 受热时将发生脱羧反应生成 CO<sub>2</sub> 和 3-aminophenol。<span class="cite-ref"><sup>[24]</sup></span> 此药物分子可互变异构为 β-酮式，则很容易发生上文讨论过的协同机理脱羧反应。Scheme 4.12 展示了此过程。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.12.png" alt="Scheme 4.12  " /><p class="caption"><span class="pic-ref">Scheme 4.12</span>  </p>
</div>
</td></tr>
<tr><td>

<p>Diflunisal ( 4-(2,4-difluoro)salicylic acid, Figure 4.2 ), structurally analogous to 4-aminosalicylic acid, also contains a β-hydroxyl group on the phenyl ring which can tautomerize as well to facilitate decarboxylation.<span class="cite-ref"><sup>[25]</sup></span></p>
</td><td>

<p>Diflunisal（或称 4-(2,4-difluoro)salicylic acid），与 4-aminosalicylic acid 结构相似，同样包含 β-酚羟基，也容易发生脱羧反应。<span class="cite-ref"><sup>[25]</sup></span></p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/F4.2.png" alt="Figure 4.2   Diflunisal 的结构式。 Diflunisal ( 4-(2,4-difluoro)salicylic acid )." /><p class="caption"><span class="pic-ref">Figure 4.2</span>   Diflunisal 的结构式。<br /> Diflunisal ( 4-(2,4-difluoro)salicylic acid ).</p>
</div>
</td></tr>
<tr><td>

<p>On the other hand, 5-aminosalicylic acid, which is also an NSAID, does not readily undergo decarboxylation, despite being structurally related to 4-amino-salicylic acid. Instead, 5-aminosalicylic acid was shown to degrade primarily via oxidative pathways.<span class="cite-ref"><sup>[25]</sup></span> This is due to the fact that 5-aminosalicylic acid contains a 4-aminophenol moiety which can readily autooxidize to produce a substituted 1,4-quinoneimine intermediate (for a detailed discussion of autooxidation, see Chapter 3, Oxidative Degradation). The latter can be further decomposed by at least two pathways: (1) hydrolysis of the imine moiety leads to the formation of gentisic acid, and (2) attack by the active pharmaceutical ingredient (API) via a Michael-type addition results in the formation of a dimer which can further react with the 1,4-quinoneimine intermediate to produce oligomers (Scheme 4.13).</p>
</td><td>

<p>另一方面，另一个非甾体类抗炎药 5-aminosalicylic acid，虽然与 4-amino-salicylic acid 结构相关却不怎么容易发生脱羧反应。实际上，此药物分子主要发生氧化降解。<span class="cite-ref"><sup>[25]</sup></span> 这是因为 5-aminosalicylic acid 含有对氨基苯酚结构，可被自然氧化生成 1,4-quinoneimine 中间体（自然氧化的具体细节见 第三章）。此中间体有两种降解途径：(1) 亚胺水解生成 gentisic acid。(2) 与另一个 API 分子发生 Michael 型加成反应生成二聚体，二聚体可继续和 1,4-quinoneimine 中间体发生反应生成齐聚物。详见 Scheme 4.13。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.13.png" alt="Scheme 4.13  " /><p class="caption"><span class="pic-ref">Scheme 4.13</span>  </p>
</div>
</td></tr>
<tr><td>

<p>As discussed above, carboxyl-containing compounds that have a β-double bond as the &quot;activator&quot; tend to undergo decarboxylation relatively easily. Other structural moieties at the β-position such as a β-epoxide or a β-leaving group can play a similar role as the β-double bond in terms of stabilizing the cyclic transition state similar to the one shown in Scheme 4.11. Hence, initial degradation which imparts these structural moieties into a carboxyl-containing drug can induce decarboxylation as one of the degradation pathways of the drug. For example, oxidative degradation of indomethacin, another NSAID, is believed to go through initial epoxidation on the 2,3-double bond of the indole ring.<span class="cite-ref"><sup>[26]</sup></span> From the epoxide intermediate, decarboxylation can occur as shown in Scheme 4.14.<span class="cite-ref"><sup>[27]</sup></span></p>
</td><td>

<p>如上所述，含有 β 双键作为活化基团的羧酸相对容易发生脱羧反应。若 β 位为环氧或易离去基团，同样能起到活化作用，如同 Scheme 4.11 中那样，环状过渡态会因此更稳定。因此，若某个含羧基的药物分子初步降解产生了此类结构后则可能进一步发生脱羧降解。比如，另一个非甾体类抗炎药 indomethacin 的氧化降解。人们认为吲哚环的 2,3-双键会被环氧化<span class="cite-ref"><sup>[26]</sup></span>，随后环氧中间体发生 Scheme 4.14 所示的脱羧反应<span class="cite-ref"><sup>[27]</sup></span>。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.14.png" alt="Scheme 4.14  " /><p class="caption"><span class="pic-ref">Scheme 4.14</span>  </p>
</div>
</td></tr>
<tr><td>

<p>Etodolac is another NSAID that also possesses an indole core structure. At the position β to the carboxyl group lies an ether functionality. According to a stability study performed by Lee et al., etodolac is susceptible to decarboxylation under neutral and acidic stress conditions.<span class="cite-ref"><sup>[28]</sup></span> Several possible scenarios (including pathways a and b in Scheme 4.15) were proposed by the original researchers. In pathway a, the key intermediate prior to the decarboxylation was proposed to be a carbocation, which is α to the indole ring but β to the carboxyl group, resulting from the protonation of the ether functionality and subsequent leaving as a hydroxyl group.</p>
</td><td>

<p>Etodolac 是另一个非甾体类抗炎药，同样含有吲哚环，羧基的 β 位有一个醚键。参考 Lee 等人的稳定性研究，etodolac 在中性和酸性条件下容易发生脱羧反应。<span class="cite-ref"><sup>[28]</sup></span> 原作者还提出了其他可能情况（Scheme 4.15 途径 a 和 b）。途径 a 中，氧原子质子化致使醚键水解，在吲哚环 α 位，即羧基 β 位形成碳正离子是脱羧反应的关键。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.15.png" alt="Scheme 4.15  " /><p class="caption"><span class="pic-ref">Scheme 4.15</span>  </p>
</div>
</td></tr>
<tr><td>

<p>Once the carbocation is formed, which should be stabilized by the indole ring, decarboxylation can readily occur to give the decarboxylated product (<strong>2A</strong>). Alternatively, a concerted mechanism (pathway b) was also proposed where decarboxylation occurs simultaneously when the ether functionality leaves (as a hydroxyl group) to yield <strong>2A</strong>. Acidification of <strong>2A</strong> would then lead to the formation of <strong>3</strong> and <strong>2B</strong>, respectively.</p>
</td><td>

<p>碳正离子受吲哚环的共轭作用而稳定存在，一旦形成，即可发生脱羧反应生成 <strong>2A</strong>。此外，醚键断裂和脱羧同时进行的协同机理，即途径 b 也有可能存在。途径 a 和 b 都生成 <strong>2A</strong>，随后可进一步反应生成 <strong>3</strong> 和 <strong>2B</strong>。</p>
</td></tr>
<tr><td>

<p>Based on the degradation product distributions under both the acidic and neutral conditions as presented by Lee et al., the present author believes that a third degradation pathway (pathway c) is also very likely. In this mechanism, decarboxylation would go via the stabilized six-membered transition state to form a 1,3-dipolar intermediate. Under neutral conditions, pathway c1 would predominate, giving <strong>2A</strong>. According to the experimental results,<span class="cite-ref"><sup>[28]</sup></span> <strong>2A</strong> is the predominant degradant when the overall degradation of etodolac is below 10%. Under the acidic condition, nevertheless, a 1,3-proton shift could be preferred. Alternatively, the carbanion could be quenched very quickly by nearby protons under acidic conditions with the eventual release of the proton on the oxygen, which process is equivalent to a net 1,3-proton shift. Both scenarios fit well with the observation that under acidic stress conditions, <strong>3</strong> is the predominant degradant when the overall degradation of etodolac is less than 50%.</p>
</td><td>

<p>根据 Lee 等人所做的酸性和中性条件下，降解产物不同分布情况，笔者认为还可能存在第三条降解途径(途径 c)。此机理中，脱羧经历六元环状过渡态，生成 1,3-偶极中间体。中性条件下，生成 <strong>2A</strong> 的途径 c1 占主导。根据他们的实验结果<span class="cite-ref"><sup>[28]</sup></span>，中性条件性下，降解总量低于 10% 时，<strong>2A</strong> 是最主要的降解产物。酸性条件下，则会发生 1,3-质子迁移，碳负离子会接受氧原子上的质子。这些推测与酸性条件下的降解情况符合，当总降解量低于 50% 时，<strong>3</strong> 成为主要的降解产物。</p>
</td></tr>
<tr><td>

<p>A large number of carboxyl-containing NSAIDs also undergo photo-catalyzed decarboxylation. The mechanisms of photochemical decarboxylation, which are different from those mentioned above, will be discussed in detail in Chapter 6, Photochemical Degradation.</p>
</td><td>

<p>许多含羧基的非甾体类抗炎药还可能发生光催化脱羧反应。此种降解的机理与上述机理不同，将在 第六章 做具体介绍。</p>
</td></tr>
<tr><td>

<h2 id="nucleophilic-conjugate-addition-and-retro-nucleophilic-conjugate-addition"><a href="#nucleophilic-conjugate-addition-and-retro-nucleophilic-conjugate-addition">4.3 Nucleophilic Conjugate Addition and Retro-nucleophilic Conjugate Addition</a></h2>
</td><td>

<h2 id="亲核共轭加成及其逆反应"><a href="#亲核共轭加成及其逆反应">4.3 亲核共轭加成及其逆反应</a></h2>
</td></tr>
<tr><td>

<p>Nucleophilic conjugate addition is a type of synthetically useful reaction between nucleophiles and α,β-unsaturated carbonyl and related compounds. The nucleophile can be an enolate, amine, alcohol, or thiol, while the carbonyl can be replaced by a nitro, cyano, sulfoxide, or sulfonyl group. When the nucleophile is an enolate, that nucleophilic conjugate addition is called Michael addition. For this reason, a nucleophilic conjugate addition is sometimes also referred to as a &quot;Michael-type reaction or addition&quot;; a general reaction scheme is shown in Scheme 4.16.</p>
</td><td>

<p>亲核共轭加成是亲核试剂与 α,β-不饱和羰基化合物（及其相关化合物）的反应，此反应在有机合成中非常有用。亲核试剂可以是烯醇负离子、胺、醇或硫醇。而硝基、氰基、亚砜或砜，也代替羰基。若亲核试剂为烯醇负离子，此亲核共轭加成反应则名为 Michael 加成。故此，亲核共轭加成有时也被称作是 Michael 型反应（或 Michael 型加成）。其反应机理见 Scheme 4.16。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.16.png" alt="Scheme 4.16  " /><p class="caption"><span class="pic-ref">Scheme 4.16</span>  </p>
</div>
</td></tr>
<tr><td>

<p>Since an α,β-unsaturated carbonyl compound is usually electrophilic and can potentially be mutagenic,<span class="cite-ref"><sup>[29]</sup></span> not many drugs contain such a functionality. Ethacrynic acid, a diuretic for the treatment of hypertension and congestive heart failure, has an α,β-unsaturated carbonyl moiety. It is interesting to note that this functionality is critical to the therapeutic effect of ethacrynic acid, while ticrynafen, another diuretic in the same family, does not contain this functionality. It was hypothesized that ethacrynic acid is a pro-drug and its adduct with cysteine, produced in vivo by a process involving initial nucleophilic conjugate addition by glutathione, is the active form.<span class="cite-ref"><sup>[30]</sup></span> Nevertheless, this hypothesis was questioned by Koechel,<span class="cite-ref"><sup>[31]</sup></span> based on the fact that not only does the in vitro reaction between ethacrynic acid and cysteine (i.e. via the expected nucleophilic conjugate addition) occur extremely fast,<span class="cite-ref"><sup>[32]</sup></span> but also so does the reverse reaction (retro-nucleophilic conjugate addition) under specific in vitro conditions.<span class="cite-ref"><sup>[33,34]</sup></span> These observations show that ethacrynic acid and its cysteine adduct are readily interconvertible. Hence, the possibility that the role of the ethacrynic acid-cysteine adduct is to transport and then release ethacrynic acid at the site of action cannot be ruled out. All of the above pathways are summarized in Scheme 4.17.</p>
</td><td>

<p>由于 α,β-不饱和碳基化合物往往是亲电性的，且有潜在的诱变性<span class="cite-ref"><sup>[29]</sup></span>，只有少数药物分子具有这种结构。用于治疗高血压和充血性心脏衰竭的利尿剂 ethacrynic acid 就含有 α,β-不饱和羰基结构。值得注意的是，此结构是药效的关键；而另一个同类药物 ticrynafen 却不含有此结构。有一种假设，ethacrynic acid 是一种前药，在体内代谢中，与谷胱甘肽发生亲核共轭加成后代谢生成的半胱氨酸加成产物才是真正的活性形式。<span class="cite-ref"><sup>[30]</sup></span> 但是，此假设受到 Koechel 的质疑<span class="cite-ref"><sup>[31]</sup></span>，这基于如下事实：在体内代谢中 ethacrynic acid 与半胱氨酸的反应（例如，上述的亲核共轭加成）速度极快<span class="cite-ref"><sup>[32]</sup></span>，且在某些特定体内条件下还会发生其逆反应(逆亲核共轭加成)<span class="cite-ref"><sup>[33,34]</sup></span>。这些观测结果说明 ethacrynic acid 与其半胱氨酸加成物可互相转化。但是，无法排除这一种可能性：即加成物转运药物分子到活性部位后释放。上述转化过程见 Scheme 4.17。</p>
</td></tr>
<tr><td>

<p>The same type of nucleophilic conjugate addition was also observed in a stability study of ethacrynic acid in aqueous solutions, in which both water and ammonium/ammonia can add onto the drug molecule.<span class="cite-ref"><sup>[35]</sup></span> In the former case, the resulting β-hydroxyl degradant further decomposed to produce two additional degradants via a retro-aldol condensation and subsequent Michael addition with another molecule of ethacrynic acid (Scheme 4.18).</p>
</td><td>

<p>在 ethacrynic acid 的水溶液稳定性研究中也观测到了相同类型的亲核共轭加成反应，水分子、铵根或氨分子可加成到药物分子上。<span class="cite-ref"><sup>[35]</sup></span> 若为水分子，所生成的 β-羟基化合物将进一步降解生成两个降解产物。涉及的反应有逆羟醛缩合以及和另一个 ethacrynic acid 分子发生 Michael 反应，详见 Scheme 4.18。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.18.png" alt="Scheme 4.18  " /><p class="caption"><span class="pic-ref">Scheme 4.18</span>  </p>
</div>
</td></tr>
<tr><td>

<p>Maleic acid, frequently used to form salts with basic drugs, is an α,β-unsaturated dicarboxylic acid. In the presence of a primary or secondary amine drug, nucleophilic conjugate addition can occur between the amine drug and maleate. For example, in several common cold medications containing phenylephrine hydrochloride and chlorpheniramine maleate or dexbrompheniramine maleate, a major degradant that formed between maleate and phenylephrine (a secondary amine) via nucleophilic conjugate addition was observed by Marin et al.<span class="cite-ref"><sup>[36]</sup></span> and Wong et al.,<span class="cite-ref"><sup>[31]</sup></span> respectively, and its structure was correctly identified by the latter group (Scheme 4.19).</p>
</td><td>

<p>马来酸(maleic acid)是一个 α,β-不饱和羧酸，常常用来与碱性药物成盐。若药物分子中存在伯胺或仲胺，则可能与马来酸发生亲核共轭加成。例如，多种抗感冒药中含有 phenylephrine hydrochloride、chlorpheniramine maleate 或 dexbrompheniramine maleate。Marin 等人<span class="cite-ref"><sup>[36]</sup></span>和 Wong 等人<span class="cite-ref"><sup>[31]</sup></span>分别发现主要降解产物是 Phenylephrine 与马来酸根生成亲核共轭加成反应产生的。后一个研究小组还确定了此降解产物的结构。详见 Scheme 4.19。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.19.png" alt="Scheme 4.19  " /><p class="caption"><span class="pic-ref">Scheme 4.19</span>  </p>
</div>
</td></tr>
<tr><td>

<p>When the nucleophile in nucleophilic conjugate addition is an amine, the resulting product is a β-aminoketone. The latter compound is also called a Mannich base because it may also be synthesized by the Mannich reaction, a synthetically versatile procedure involving three starting materials: a primary or secondary amine, a non-enolizable aldehyde, and an enolate.</p>
</td><td>

<p>若参与反应的亲核试剂是胺，则生成 β-氨基酮。此化合物是一个 Mannich 碱，这是因为此种结构在有机合成中通常会使用 Mannich 反应构建。Mannich 反应是一个三组分反应：伯胺或仲胺、未烯醇化的醛、烯醇负离子。</p>
</td></tr>
<tr><td>

<p>β-Aminoketones are susceptible to degradation via retro-nucleophilic conjugate addition, which is also a type of elimination reaction. For example, the muscle relaxants eperisone and tolperisone are Mannich bases. Under both stress and ambient storage conditions, the drugs were found to degrade via the retro-nucleophilic conjugate addition mechanism (Scheme 4.20).<span class="cite-ref"><sup>[38,39]</sup></span> The degradation occurred in neutral to basic solutions and the rate of the degradation was accelerated as the pH increased, which is consistent with the mechanism shown above. Dyclonine in Scheme 4.20 above is another β-aminoketone drug that degrades via the same retro-nucleophilic conjugate addition.<span class="cite-ref"><sup>[40]</sup></span></p>
</td><td>

<p>β-氨基酮容易发生逆亲核共轭加成反应而降解，这是一种消除反应。例如，肌肉松弛剂 eperisone 和 tolperisone 都是 Mannich 碱。在强制降解和常温贮藏条件下，这两者都会发生逆亲核共轭加成反应而降解(Scheme 4.20)。<span class="cite-ref"><sup>[38,39]</sup></span> 中性或碱性条件会发生种降解，且 pH 升高时降解速度提高，这符合上述机理。Scheme 4.20 中的 Dyclonine 是另一具有 β-氨基酮结构的药物，可发生同样的逆亲核共轭加成反应。<span class="cite-ref"><sup>[40]</sup></span></p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.20.png" alt="Scheme 4.20  " /><p class="caption"><span class="pic-ref">Scheme 4.20</span>  </p>
</div>
</td></tr>
<tr><td>

<h2 id="aldol-condensation-and-retro-aldol"><a href="#aldol-condensation-and-retro-aldol">4.4 Aldol Condensation and Retro-aldol</a></h2>
</td><td>

<h2 id="羟醛缩合及其逆反应"><a href="#羟醛缩合及其逆反应">4.4 羟醛缩合及其逆反应</a></h2>
</td></tr>
<tr><td>

<h3 id="aldol-condensation"><a href="#aldol-condensation">4.4.1 Aldol Condensation</a></h3>
</td><td>

<h3 id="羟醛缩合"><a href="#羟醛缩合">4.4.1 羟醛缩合</a></h3>
</td></tr>
<tr><td>

<p>Aldol condensation is an important organic reaction between an enol or enolate and a non-conjugated ketone or aldehyde. Since enol and enolate are generated from a ketone/aldehyde, aldol condensation can take place between the same ketone/aldehyde molecules, as well as between different ketone/aldehyde molecules (Scheme 4.21).</p>
</td><td>

<p>羟醛缩合是另一个重要的有机反应，是烯醇或烯醇负离子与非共轭醛/酮发生的反应。而烯醇或烯醇负离子一般由醛/酮产生，故此两个醛/酮分子间就可以发生羟醛缩合，可能是相同的两个醛/酮分子，也可能不同。详见 Scheme 4.21。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.21.png" alt="Scheme 4.21  " /><p class="caption"><span class="pic-ref">Scheme 4.21</span>  </p>
</div>
</td></tr>
<tr><td>

<p>The reaction is catalyzed by both acid and base, and the resulting β-hydroxyketo intermediate (i.e. an aldol) usually undergoes dehydration to yield the final, more stable α,β-unsaturated keto product.</p>
</td><td>

<p>此反应可受酸或碱催化，会生成 β-羟基酮(或称 aldol)中间体，一般会脱水生成较稳定的最终产物 α,β-不饱和醛/酮。</p>
</td></tr>
<tr><td>

<p>Although synthetically a well utilized methodology, aldol condensation does not appear to be a very common pathway in drug degradation, in particular in condensation between molecules of the same carbonyl compound. One example can be found in a veterinary antibiotic drug, tylosin. Tylosin is produced by fermentation and its main component, tylosin A, and most other related macrolides, contain an aldehyde group in the core macrocyclic ring. In a stability study of tylosin by Fish and Carr, a major degradant was observed in a liquid formulation with a pH of 9.<span class="cite-ref"><sup>[41]</sup></span> The degradation was attributed to the aldol condensation of the drug substance, although no specific structure of the degradant was shown. Nevertheless, it is conceivable that the condensation occurs between the aldehyde group and the enol/enolate generated on the methylene group α to the aldehyde functionality of another tylosin molecule (Figure 4.3).</p>
</td><td>

<p>虽然在有机合成中很有用，但羟醛缩合在药物降解中却并不常见，特别是相同的两个羰基化合物之间的缩合更是少见。有一个例子是兽用药抗生素 tylosin。此药物由发酵生产，其主要组分是 tylosin A 和多数其他相关的大环内酯类化合物一样，内酯母核上有醛基。Fish 和 Carr 对进行了稳定性研究，在 pH 9 的液体制剂中观测到了主要降解产物。<span class="cite-ref"><sup>[41]</sup></span> 降解被认为与药物分子间的羟醛缩合有关，但并没有给出降解产物的确定结构。然而，可以想象得到，缩合发生于醛基和另一个 tylosin 分子中醛基 α 位的亚甲基上形成的烯醇/烯醇负离子之间(Figure 4.3)。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/F4.3.png" alt="Figure 4.3   Tylosin A." /><p class="caption"><span class="pic-ref">Figure 4.3</span>   Tylosin A.</p>
</div>
</td></tr>
<tr><td>

<p>Haloperidol is an antipsychotic drug in the butyrophenone family containing a phenyl-conjugated ketone functionality. During a comparability study with lactose, it was found to form an aldol condensation product with a furanaldehyde, 5-(hydroxymethyl)-2-furaldehyde (5-HMF),<span class="cite-ref"><sup>[42]</sup></span> an impurity found in a number of sugars including lactose.<span class="cite-ref"><sup>[43,44]</sup></span> The degradation reaction is shown in Scheme 4.22.</p>
</td><td>

<p>Haloperidol 是丁酰苯类抗精神分裂药，它含有与苯环共轭的羰基。在与乳酸的相容性研究中发现，它会与糖类中常见的杂质 5-羟甲基糠醛(furanaldehyde, 5-(hydroxymethyl)-2-furaldehyde, 5-HMF)<span class="cite-ref"><sup>[43,44]</sup></span>发生羟醛缩合反应。<span class="cite-ref"><sup>[42]</sup></span> 此降解反应见 Scheme 4.22。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.22.png" alt="Scheme 4.22  " /><p class="caption"><span class="pic-ref">Scheme 4.22</span>  </p>
</div>
</td></tr>
<tr><td>

<p>Ziprasidone is another antipsychotic drug that has a lactam rather than a ketone functionality. During a formulation study with various cyclodextrins and their derivatives, Hong et al. found that aldol condensation was involved in the degradation of the formulated ziprasidone both in solution and lyophilized amorphous solid.<span class="cite-ref"><sup>[45]</sup></span> What happened was that the methylene moiety next to the lactam carbonyl was first oxidized to form a new carbonyl group. This carbonyl is more reactive than a regular ketone owing to activation by the neighboring lactam carbonyl group. Hence, this oxidative degradant was capable of aldol condensation with ziprasidone in the formulation. The final degradant was found to be in the E-configuration as illustrated in Scheme 4.23.</p>
</td><td>

<p>Ziprasidone 是另一个抗精神分裂药，但它不含羰基，而是内酰胺。Hong 等人在制剂研究使用了多种环糊精及其衍生物，他们发现在液体剂型或冻干粉中都羟醛缩合产物生成。<span class="cite-ref"><sup>[45]</sup></span> 首先内酰胺的 α 位亚甲基被氧化为羰基，由于酰胺键的存在此羰基的反应活性比一般的羰基要高。于是，此降解产物可以和制剂中的另一个 ziprasidone 分子发生羟醛缩合。最终的降解产物为顺式，反应见 Scheme 4.23。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.23.png" alt="Scheme 4.23  " /><p class="caption"><span class="pic-ref">Scheme 4.23</span>  </p>
</div>
</td></tr>
<tr><td>

<h3 id="retro-aldol-reaction"><a href="#retro-aldol-reaction">4.4.2 Retro-aldol Reaction</a></h3>
</td><td>

<h3 id="逆羟醛缩合"><a href="#逆羟醛缩合">4.4.2 逆羟醛缩合</a></h3>
</td></tr>
<tr><td>

<p>As discussed above, the β-hydroxyketone intermediate in aldol condensation usually undergoes dehydration, if an α-hydrogen is present. On the other hand, it can also revert to the two starting molecules, that is, undergo a retro-aldol reaction. Hence, drug molecules or their intermediary degradants containing a β-hydroxyketone moiety are susceptible to degradation via the retro-aldol pathway, which has been observed in a few cases. For example, basic stress of corticosteroids containing a 1,3-dihydroxylacetone side chain on the D-ring, such as prednisolone,<span class="cite-ref"><sup>[46]</sup></span> hydrocortisone,<span class="cite-ref"><sup>[47]</sup></span> and betamethasone<span class="cite-ref"><sup>[3]</sup></span> under anaerobic conditions leads to the formation of the corresponding corticosteroid 17-ketones. This degradation process has been attributed to retro-aldol reaction after the drug substances tautomerize to produce the β-hydroxyketone moiety as depicted in Scheme 4.24.<span class="cite-ref"><sup>[48,49]</sup></span> In the presence of oxygen, the formation of the corticosteroid 17-ketone could also stem from the retro-aldolization of those oxidative degradants that contain a β-hydroxyketone moiety.<span class="cite-ref"><sup>[50]</sup></span></p>
</td><td>

<p>上文已经介绍过，若存在 α-氢，羟醛缩合中生成的 β-羟基酮中间体一般会发生失水反应。另一方面，它也可能分解而给出缩合前的那两个分子，即逆羟醛缩合反应。因此，在某些特殊情况下，含有 β-羟基酮结构的药物分子或降解中间体也能发生相应的逆羟醛缩合反应。例如，D 环侧链中含有 1,3-二羟基酮结构的某些皮质类固醇药物，比如 prednisolone<span class="cite-ref"><sup>[46]</sup></span>、hydrocortisone<span class="cite-ref"><sup>[47]</sup></span> 和 betamethasone<span class="cite-ref"><sup>[3]</sup></span>，在无氧条件下可形成相应的 corticosteroid 17-ketone。药物分子互变异构为 β-羟基酮，然后发生逆羟醛缩合反应。此降解过程展示于 Scheme 4.24。<span class="cite-ref"><sup>[48,49]</sup></span> 当氧气存在时，corticosteroid 17-ketone 还可能由其他含有 β-羟基酮结构的氧化降解产物生成。</p>
</td></tr>
<tr><td>

<p>Ginger has not only been widely used as a spice, in particular in Asian cuisines, but also as a folk medicine since ancient times. 6-Gingerol, the major pungent principle isolated from ginger oleoresin, possesses a β-hydroxyketo moiety and hence is susceptible to retro-aldol reaction to yield zingerone (Scheme 4.25).<span class="cite-ref"><sup>[51]</sup></span></p>
</td><td>

<p>姜是广泛使用的调料，尤其是在亚洲烹饪更是频繁使用。自古以来，它还是一种民间药物。由姜油树脂中分离出的 6-Gingerol 是其辛辣气味的主要来源，其分子中含有 β-羟基酮结构，因此它容易发生逆羟醛缩合反应而生成 zingerone (Scheme 4.25)。<span class="cite-ref"><sup>[51]</sup></span></p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.25.png" alt="Scheme 4.25  " /><p class="caption"><span class="pic-ref">Scheme 4.25</span>  </p>
</div>
</td></tr>
<tr><td>

<h2 id="isomerization-and-rearrangement"><a href="#isomerization-and-rearrangement">4.5 Isomerization and Rearrangement</a></h2>
</td><td>

<h2 id="异构化和重排"><a href="#异构化和重排">4.5 异构化和重排</a></h2>
</td></tr>
<tr><td>

<p>Isomerization is the process in which a molecule is transformed into another chemical entity with the same chemical formula. Rearrangement refers to any organic reaction in which the carbon framework of a reactant is rearranged to produce a product that is isomeric with the original molecule. According to this definition, rearrangement can be considered a subset of isomerization. Nevertheless, a broader definition of rearrangement is sometimes used, whereby the rearranged product is not isomeric but largely similar in its formula to the original molecule. A vast number of chemical transformations fall into the definitions of isomerization and rearrangement, which will be discussed in the following sections.</p>
</td><td>

<p>异构化反应，指某种化学物质改变自身的组成结构，从而成为新物质的反应。重排反应是分子的碳骨架发生重排生成结构异构体的化学反应。基于上述定义，重排反应可看做是异构化反应的一个特例。但有时则指广义的重排反应，此时降解产物不是反应物的异构体而只是大部分相似。大量化学转化都可以归类于异构化和重排反应，我们将在下文详细讨论。</p>
</td></tr>
<tr><td>

<h3 id="tautomerization"><a href="#tautomerization">4.5.1 Tautomerization</a></h3>
</td><td>

<h3 id="互变异构"><a href="#互变异构">4.5.1 互变异构</a></h3>
</td></tr>
<tr><td>

<p>Tautomerization is a key process involved in many chemical transformations and/or mechanisms, including various isomeric degradation pathways. The examples include the formation of enol/enolate in an aldol condensation and imine-enamine tautomerization. Frequently, tautomers, in particular intermediary tautomers, may not be isolatable and/or observable chromatographically. Nevertheless, in certain cases, the energy barrier between the tautomers is large enough to enable their isolation. For example, it was found that the main degradation product of cefpodoxime proxetil in both the solid and solution states is a tautomeric degradant resulting from a double bond shift inside the six-membered ring of the cephalosporin core structure (Scheme 4.26).<span class="cite-ref"><sup>[52]</sup></span></p>
</td><td>

<p>互变异构是许多化学转化过程或机理的关键，往往包含多种异构化降解途径。例如，羟醛缩合中涉及的烯醇/烯醇负离子的形成、亚胺-烯胺的互变异构。互变异构体，特别是那些反应中间体，一般是无法被分离或利用色谱手段观测到的。此外，在某些特例中，互变异构体之间的能垒足够大时则有可能分离。例如， cefpodoxime proxetil 在固体或溶液状态下主要降解产物是六元环母核上的双键移位而生成的互变异构体(Scheme 4.26)。<span class="cite-ref"><sup>[52]</sup></span></p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.26.png" alt="Scheme 4.26  " /><p class="caption"><span class="pic-ref">Scheme 4.26</span>  </p>
</div>
</td></tr>
<tr><td>

<p>When in solution, it was found that the rate of isomerization increased as the pH decreased. The isomerization is likely to proceed through the enol intermediate as illustrated above.</p>
</td><td>

<p>在溶液中，随 pH 降低，异构化反应速度相应提高。如上图所示，此异构化似乎是经历了烯醇中间体。</p>
</td></tr>
<tr><td>

<h3 id="racemization"><a href="#racemization">4.5.2 Racemization</a></h3>
</td><td>

<h3 id="消旋化"><a href="#消旋化">4.5.2 消旋化</a></h3>
</td></tr>
<tr><td>

<p>A racemic mixture is a collection of two enantiomers and racemization is a process of chemical transformation in which one enantiomer converts to another. The only meaningful racemization, in terms of drug degradation, is usually one related to compounds that contain a single chiral center. For compounds containing multiple chiral centers, conversion of one chiral center results in the formation of a diastereomer, a process called epimerization which will be discussed in the next section.</p>
</td><td>

<p>外消旋混合物是一对对映异构体的混合物；消旋化是指某个旋光化合物在一定条件下转变为对应异构体的过程。在药物降解中，唯一有意义的消旋化只针对于只含有一个手性中心的药物分子。若含有多个手性中心，单个手性中性的构型翻转会形成非对应异构体，此过程名为差向异构化，我们将在下一节中讨论。</p>
</td></tr>
<tr><td>

<p>Paliperidone, or 9-hydroxyrisperidone, is an active oxidative metabolite of risperidone. The hydroxylation creates a chiral center at the 9-position of paliperidone. Despite the two enantiomers of paliperidone having comparable pharmacological activity, they display different affinities in plasma protein binding.<span class="cite-ref"><sup>[53]</sup></span> Danel et al. studied the configuration stability of paliperidone and found that the drug substance undergoes racemization under both acidic and basic conditions, but a faster rate of racemization was observed under acidic conditions.<span class="cite-ref"><sup>[54]</sup></span> The mechanism of racemization was found to be mediated by an imine-enamine tautomerization process (Scheme 4.27), according to evidence from a H/D exchange study via NMR analysis. Under the physiological condition, no racemization of paliperidone was observed.</p>
</td><td>

<p>Paliperidone 又名 9-hydroxyrisperidone，是 risperidone 的体内活性代谢产物。水解使得 paliperidone 的 9 位成为手性中心。尽管 paliperidone 的两种对映体具有类似的药理活性，但它们与血浆蛋白结合的亲和力却不同。<span class="cite-ref"><sup>[53]</sup></span> Danel 等人对 paliperidone 的构型稳定性进行了研究，结果发现，药物分子在的酸性和碱性条件下都会发生外消旋化，但在酸性条件下外消旋化的速度更快。<span class="cite-ref"><sup>[54]</sup></span> 由 H/D 同位素交换和核磁共振分析研究后发现，消旋化经历亚胺/烯胺互变异构化(Scheme 4.27)。在生理条件下，未观察任何外消旋化。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.27.png" alt="Scheme 4.27  " /><p class="caption"><span class="pic-ref">Scheme 4.27</span>  </p>
</div>
</td></tr>
<tr><td>

<p>Epinephrine, also called adrenaline, is a therapeutic agent commonly used in emergency medicine to treat cardiac arrest and other cardiac dysrhythmias. The drug is typically administrated by intravenous injection, and since it is chemically a catecholamine, it is usually formulated with excipients that have antioxidation capability, such as sodium metabisulfite, in order to suppress its rather facile autooxidation. In a liquid formulation, epinephrine was found to undergo sulfonation and subsequent racemization via carbocation through an SN<sub>1</sub> mechanism (Scheme 4.28).<span class="cite-ref"><sup>[55]</sup></span></p>
</td><td>

<p>Epinephrine 又名 adrenaline，是常用于治疗心跳骤停和心律不齐的急救药物。此药一般通过静脉内注射给药，从化学上讲它是一个儿茶酚胺(Catecholamine)。为了抑制极容易发生的自然氧化反应，在制剂中通常要加入具有抗氧化能力的赋形剂，如偏亚硫酸氢钠。在液体制剂中，epinephrine 会形成碳正离子，随后经历 SN<sub>1</sub> 机理发生磺化和消旋化。<span class="cite-ref"><sup>[55]</sup></span> 见 Scheme 4.28。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.28.png" alt="Scheme 4.28  " /><p class="caption"><span class="pic-ref">Scheme 4.28</span>  </p>
</div>
</td></tr>
<tr><td>

<p>The SN1 mechanism is probably made possible by a combination of the following two factors: first, sulfonate is an excellent leaving group and second, the carbocation formed can be stabilized through resonance with the adjacent catechol moiety. The D- or S-epinephrine resulting from the racemization is essentially inactive.</p>
</td><td>

<p>受以下两个因素的影响，SN<sub>1</sub> 机理成为可能：首先，磺酸根是一个很好的离去基团。第二，所形成的碳正离子与苯环共轭而得以稳定。消旋化生成的 D-epinephrine (或称 R-epinephrine) 没有任何生物活性。</p>
</td></tr>
<tr><td>

<h3 id="epimerization"><a href="#epimerization">4.5.3 Epimerization</a></h3>
</td><td>

<h3 id="差向异构化"><a href="#差向异构化">4.5.3 差向异构化</a></h3>
</td></tr>
<tr><td>

<p>Epimerization relates to compounds with multiple chiral centers. Epimers are those diastereomers that differ in configuration only at a single chiral center. The anticancer drug etoposide is a semi-synthetic epipodophyllotoxin derivative that contains multiple chiral centers. The chiral center α to its lactone carbonyl group is susceptible to racemization, which was found to be mediated by enolization at pH above 4.<span class="cite-ref"><sup>[56,57]</sup></span> The mechanism of the enolization-mediated epimerization is shown in Scheme 4.29.</p>
</td><td>

<p>差向异构化是具有多个手性中心的化合物才有的反应。一对非对映异构体，若只有一个手性中心的构型有所不同，则称为差向异构体。抗癌剂 etoposide 是一种半合成而得的 epipodophyllotoxin 衍生物，含有多个手性中心。位于内酯的羰基 α 位的手性中心在 pH 大于 4 时会发生烯醇化，进而发生消旋化。<span class="cite-ref"><sup>[56,57]</sup></span> Scheme 4.29 展示了此差向异构化反应的机理。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.29.png" alt="Scheme 4.29  " /><p class="caption"><span class="pic-ref">Scheme 4.29</span>  </p>
</div>
</td></tr>
<tr><td>

<h4 id="cis-trans-isomerization"><a href="#cis-trans-isomerization">4.5.4 Cis-trans Isomerization</a></h4>
</td><td>

<h4 id="顺反异构化"><a href="#顺反异构化">4.5.4 顺反异构化</a></h4>
</td></tr>
<tr><td>

<p>Oxime including oxime ether is a common structural moiety utilized in drug design and optimization. Drug molecules possessing this moiety are susceptible to cis-trans (also called syn-anti) isomerization around the oxime double bond. The isomerization is reversible and the more stable trans-isomer (E-isomer) usually predominates. The isomerization process can proceed via two competing pathways as illustrated in Scheme 4.30. The first is electron doublet inversion and the second catalyzed rotation involving oxime/nitroso tautomerization in both acidic and basic media.<span class="cite-ref"><sup>[58]</sup></span></p>
</td><td>

<p>肟以及肟醚都是是药物设计和优化中常用的结构。这些药物分子中，肟的双键很容易发生顺反异构（cis-trans 或 syn-anti isomerization）。此异构化反应是可逆的，反式异构体(E-异构体)更稳定些，通常是主要的存在形式。此异构化反应可通过两个彼此竞争的途径进行，如 Scheme 4.30 所示。首先，电子双重反转；然后，经历肟/亚硝基的互变异构体发生翻转。在酸性和碱性介质中都是如此。<span class="cite-ref"><sup>[58]</sup></span></p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.30.png" alt="Scheme 4.30  " /><p class="caption"><span class="pic-ref">Scheme 4.30</span>  </p>
</div>
</td></tr>
<tr><td>

<p>Roxithromycin (Figure 4.4) is an oxime ether derivative of erythromycin whose oxime moiety undergoes cis-trans isomerization at pH below 5 to give the less potent Z-isomer.<span class="cite-ref"><sup>[59]</sup></span> The ratio of roxithromycin (the E-isomer) versus the Z-isomer appeared to be constant at a particular pH value. Between pH 1 and 3, the isomeric degradation followed pseudo first-order kinetics and the rate of isomerization and the Z/E ratio increased as the pH decreased.</p>
</td><td>

<p>Figure 4.4 为 Roxithromycin 的结构式，这是 erythromycin 的一个肟醚衍生物。pH 小于 5 时，它可以发生顺-反式异构化，生成活性较弱的 Z-异构体。<span class="cite-ref"><sup>[59]</sup></span>在特定的 pH 下，roxithromycin (E-异构体) 与其 Z-异构体的比例是一个定值。pH 处于 1 和 3 之间时，随着 pH 下降，异构化呈准一级动力学特征，反应速率、Z/E 异构体比例相应增大。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/F4.4.png" alt="Figure 4.4   Roxithromycin 结构式。 Roxithromycin." /><p class="caption"><span class="pic-ref">Figure 4.4</span>   Roxithromycin 结构式。<br /> Roxithromycin.</p>
</div>
</td></tr>
<tr><td>

<p>A fairly large number of antibiotics in the cephalosporin family, including cefdinir, cefixime, cefpodoxime, ceftizoxime, and cefmenoxime, contain an oxime/oxime ether moiety and hence should be susceptible to the same degradation via the cis-trans isomerization mechanism discussed here.</p>
</td><td>

<p>相当多的头孢类抗生素，比如cefdinir、cefixime、cefpodoxime、ceftizoxime 和 cefmenoxime，其分子中都包含肟/肟醚结构，因此都有可能经历上述机理而发生顺反异构化。</p>
</td></tr>
<tr><td>

<p>Compounds containing polyconjugated double bonds undergo cis-trans isomerization both photochemically and non-photochemically. As photochemical isomerization will be covered in Chapter 6, Photochemical Degradation, only non-photochemical isomerization is discussed in this section.</p>
</td><td>

<p>含有多个共轭双键的化合物也能发生顺反异构化，可能的机理包括光化学机理和常规机理。而光化学异构化将在第六章详细讨论，本小节则只介绍光化学范畴之外的异构化反应。</p>
</td></tr>
<tr><td>

<p><strong>Ro-26-9228</strong> is a vitamin D derivative, structurally similar to calcitriol, the hormonally active form of vitamin D. Like calcitriol,<span class="cite-ref"><sup>[60]</sup></span> Ro-26-9228 also undergoes conformational as well as chemical cis-trans isomerization.<span class="cite-ref"><sup>[61]</sup></span> The chemical cis-trans isomerization was shown to proceed through a {1,7}-hydrogen shift in the minor Z-conformer as depicted in Scheme 4.31.</p>
</td><td>

<p><strong>Ro-26-9228</strong> 是维生素D 的衍生物，其结构类似骨化三醇，而骨化三醇是维生素D 的活性形式，也是体内的一种激素。<span class="cite-ref"><sup>[60]</sup></span> 与骨化三醇一样，<strong>Ro-26-9228</strong> 也会发生构象异构和顺反异构化。<span class="cite-ref"><sup>[61]</sup></span> 构化生成 Z-异构体，但会进一步发生 {1,7}-氢转移，故此体系中最终只存在少量的 Z-异构体。详见 Scheme 4.31。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.31.png" alt="Scheme 4.31  " /><p class="caption"><span class="pic-ref">Scheme 4.31</span>  </p>
</div>
</td></tr>
<tr><td>

<p>Drugs possessing carbon-carbon double bonds conjugated to a carbon-hetero atom double bond are also susceptible to geometric isomerization. For example, ceftibuten, another antibiotic in the cephalosporin family, is structurally unique in its C-7 side chain as compared to the vast majority of cephalosporin antibiotics in that the typical oxime/oxime ether moiety of the latter is replaced by a carbon-carbon double bond. The latter double bond is conjugated to both the 7-amide carbonyl group and aminothiazole ring. It was found that ceftibuten undergoes cis-trans isomerization around the conjugated carbon-carbon double bond in both acidic and basic solutions.<span class="cite-ref"><sup>[62]</sup></span> There was no detailed discussion regarding the mechanism of the isomerization in the original study. However, based on the structure of ceftibuten, it appears that tautomerization, which can be trigged by deprotonation of the methylene group α to the conjugated carbon-carbon double bond, could be responsible for the isomeric degradation as shown in Scheme 4.32.</p>
</td><td>

<p>若分子中存在碳-碳双键与碳-杂原子双键共轭，则也容易发生几何异构化。例如，头孢类抗生素 ceftibuten，相比于其他同类抗生素，其 C-7 位侧链的结构独特，不再含有经典的肟/肟醚结构，而是代之以碳-碳双键。此碳-碳双键与 7 位酰胺的羰基、噻唑环构成共轭。实验发现，在酸性或碱性溶液中，此碳-碳双键会发生顺反异构化反应。<span class="cite-ref"><sup>[62]</sup></span> 在原研究中没有提及异构化反应的详细机理。然而，结合 cefibuten 的结构可以预见，碳-碳双键 α 位的亚甲基发生去质子化，双键移位生成多个互变异构体，最终产生 E-异构体。见 Scheme 4.32。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.32.png" alt="Scheme 4.32  " /><p class="caption"><span class="pic-ref">Scheme 4.32</span>  </p>
</div>
</td></tr>
<tr><td>

<p>The proposed mechanism seems to be consistent with the experimental results presented by the original researchers. For example, several ceftibuten analogs, in which the aminothiazole ring was replaced by rings with fewer hetero atoms (e.g. phenyl ring), were also prepared and their isomerization behavior was compared with that of ceftibuten. These analogs all displayed much reduced rates of isomerization under acidic conditions, probably due to their inability to form as many tautomers as ceftibuten, which makes it more difficult for the tautomerization process to take place.</p>
</td><td>

<p>此机理基本符合原作者的实验结果。例如，他们将氨基噻唑环替换为其他杂原子更少的芳香环（比如苯环），从而得到了 ceftibuten 的类似物，然后研究了它们的异构化反应作为比较。研究发现，类似物的异构化反应速率大幅降低。可能是由于这些类似物无法形成 ceftibuten 那样的互变异构体，它们在酸性条件下更难发生异构化反应。</p>
</td></tr>
<tr><td>

<h3 id="no-acyl-migration"><a href="#no-acyl-migration">4.5.5 N,O-Acyl Migration</a></h3>
</td><td>

<h3 id="no-酰基迁移"><a href="#no-酰基迁移">4.5.5 N,O-酰基迁移</a></h3>
</td></tr>
<tr><td>

<p>Cyclosporin A is a widely used immunosuppressive drug for patients after organ and tissue transplantation. It has a cyclic undecapeptide structure and the side chain hydroxyl group of one unusual amino acid residue can attack the carbonyl group of the neighboring valine under acidic conditions. This N,O-acyl migration results in the formation of isocyclosporin A (Scheme 4.33).<span class="cite-ref"><sup>[63,64]</sup></span> The impact of this isomerization arising from possible drug degradation in the stomach was assessed.<span class="cite-ref"><sup>[64]</sup></span> Based on its half-lives at 37 °C between pH 1 to 3, it was estimated that only 1-2% of the ingested drug would undergo decomposition while passing through the stomach.</p>
</td><td>

<p>Cyclosporin A 是一种免疫抑制药物，广泛用于接受过器官或组织移植的患者。它是一个环状的十一肽，其中有一个特别的氨基酸残基，其侧链上有一个羟基。在酸性条件下，此羟基可进攻邻位缬氨酸残基的羰基。此时会发生 N,O-酰基迁移，生成 isocyclosporin A (Scheme 4.33)。<span class="cite-ref"><sup>[63,64]</sup></span> 此药物在胃中能发生降解，已确定会受到此异构化反应的影响。<span class="cite-ref"><sup>[64]</sup></span> 在 37 °C，pH 1 ~ 3 时，根据反应半衰期估算，通过胃后仅有 1-2％的药物发生了分解。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.33.png" alt="Scheme 4.33  " /><p class="caption"><span class="pic-ref">Scheme 4.33</span>  </p>
</div>
</td></tr>
<tr><td>

<p>Another example of N,O-acyl migration in drug degradation was found during the development of a water soluble derivative of camptothecin, camptothecin-20(S)-glycinate.<span class="cite-ref"><sup>[65]</sup></span> In this case, the direction of acyl migration is from O to N, which is the opposite of the previous example. At pH 7.4, 37 °C, the glycinate was found to decompose quickly with a half-life of ~30 minutes. On the other hand, the analogous 20(S)-acetate, which lacks the a-amino group, showed no sign of decomposition during a period of 3 hours under identical conditions. The instability of the 20(S)-glycinate was attributed to a sequential degradation that is triggered by an N,O-acyl migration step (Scheme 4.34).</p>
</td><td>

<p>还有另一个发生 N,O-酰基迁移的药物降解的实例，在开发一种水溶性 camptothecin 衍生物 camptothecin-20(S)-glycinate <span class="cite-ref"><sup>[65]</sup></span> 的过程中发现，酰基会从氧原子迁移到氮原子，这与前面的例子正好相反。在 pH 7.4，37 °C 时，甘氨酸会迅速分解，半衰期约为 30 分钟。另一方面，缺少 α-氨基的 20(S)-acetate 类似物，在相同的条件下处理 3 小时，仍然未发现分解的迹象。20(S)-glycinate 的不稳定性正是因为 N,O-酰基迁移，进而发生一系列降解(Scheme 4.34)。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.34.png" alt="Scheme 4.34  " /><p class="caption"><span class="pic-ref">Scheme 4.34</span>  </p>
</div>
</td></tr>
<tr><td>

<h3 id="rearrangement-via-ring-expansion"><a href="#rearrangement-via-ring-expansion">4.5.6 Rearrangement via Ring Expansion</a></h3>
</td><td>

<h3 id="扩环重排"><a href="#扩环重排">4.5.6 扩环重排</a></h3>
</td></tr>
<tr><td>

<p>It has long been known that corticosteroids containing the 17-hydroxy-20-keto moiety are susceptible to D-ring expansion, also known as D-homoannulation, under catalysis by metal ions (or Lewis acids), bases, and other factors.<span class="cite-ref"><sup>[66]</sup></span> Over the many decades that followed this study, various studies have been performed to understand the mechanism of this rearrangement reaction.<span class="cite-ref"><sup>[67-69]</sup></span> Dekker and Beijnen studied the degradation behavior of prednisolone, dexamethasone, and betamethasone at a moderately alkaline pH of 8.3 and under elevated temperature. It was found that the D-homoannulation rearrangement products were among the top three degradants of prednisolone and dexamethasone, respectively (Scheme 4.35).<span class="cite-ref"><sup>[70]</sup></span></p>
</td><td>

<p>人们很早就发现含有 17-hydroxy-20-keto 结构的皮质类固醇药物分子在金属离子（路易斯酸）、碱或其他因素影响下，其 D-环很容易发生扩环反应，即 D-homoannulation。<span class="cite-ref"><sup>[66]</sup></span> 经过数十年的研究，人们探明了此重排反应的机理。<span class="cite-ref"><sup>[67-69]</sup></span> Dekker 和 Beijnen 研究了 prednisolone、dexamethasone 和 betamethasone 在弱碱性(pH 8.3)、升温条件下的降解行为。他们发现扩环重排产物是此三者的最主要降解产物。见 Scheme 4.35。<span class="cite-ref"><sup>[70]</sup></span></p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.35.png" alt="Scheme 4.35  " /><p class="caption"><span class="pic-ref">Scheme 4.35</span>  </p>
</div>
</td></tr>
<tr><td>

<p>During these studies, the original researchers only identified the two final degradation products shown in Scheme 4.35. Based on current knowledge, the D-homoannulation of prednisolone and dexamethasone should first give rise to their respective ring expansion products (i.e. the two intermediates in Scheme 4.35). Since the original studies were performed under a relatively high temperature of 100 °C, a retro-aldol process for the initial D-homoannulation products would be very likely to occur, which should yield the final isolated products.</p>
</td><td>

<p>在此研究中，原作者仅鉴别了 Scheme 4.35 中所示的两个最终降解产物。基于现有知识可知，prednisolone 和 dexamethasone 发生 D-homoannulation，要先生成相应的扩环产物（即 Scheme 4.35 所示的两个反应中间体）。由于原作者的实验温度相对较高(100 °C)，很容易发生逆羟醛缩合反应，故而分离得到上述降解产物。</p>
</td></tr>
<tr><td>

<p>On the other hand, no degradation product of betamethasone resulting from the D-homoannulation was observed under the same pH and stress temperature.<span class="cite-ref"><sup>[71]</sup></span> These observations are interesting in that small differences at the 16-position of the three steroids can cause a quite dramatic difference in the distribution of their degradation products.</p>
</td><td>

<p>另一方面，在相同的 pH 和 温度下，未能发现 betamethasone 的 D-homoannulation 降解产物。<span class="cite-ref"><sup>[77]</sup></span> 这是因为这三个分子在 16-位的细微差异造成了其降解行为的不同。</p>
</td></tr>
<tr><td>

<p>When catalyzed by metal ions (Lewis acids), it was proposed that the D-homoannulation proceeds through a transition complex in which the metal ion is chelated by the 17-hydroxyl and 20-keto groups, as shown in Scheme 4.36.<span class="cite-ref"><sup>[72]</sup></span> Because of the chelation, the 17-hydroxyl and 20-keto groups are locked in a syn-configuration, which enables a stereo-specific attack on the 17-keto group by the migrating alkyl group.<span class="cite-ref"><sup>[69]</sup></span> As a result, only a single diastereomeric degradant is formed during the metal ion-catalyzed D-homoannulation process.</p>
</td><td>

<p>若金属离子（路易斯酸）作为催化剂，一般认为 D-homoannulation 将经历一个螯合过渡态，如 Scheme 4.36 所示，金属离子与 17-羟基、20-酮构成螯合。<span class="cite-ref"><sup>[72]</sup></span> 由于螯合作用存在，17-羟基和 20-酮这两个基团被限制在顺式构象，这使得烷基迁移时必须立体专一地进攻 17-酮基团。<span class="cite-ref"><sup>[69]</sup></span> 故此，降解产物的手性是固定的。</p>
</td></tr>
<tr><td>

<p>Triamcinolone is a corticosteroid that is structurally identical to prednisolone except for an additional hydroxyl group at the 16α-position. Owing to the presence of the 16α-hydroxyl group, which is β to the 20-carbonyl group, triamcinolone appears to be more susceptible to D-ring expansion, consistent with previous observations that the 16,17-dihydroxy-20-keto moiety is more prone to rearrangement than the 17-hydroxy-20-keto moiety.<span class="cite-ref"><sup>[68]</sup></span> Furthermore, under base catalysis, the D-ring expansion of triamcinolone appears to proceed via a different mechanism. According to Delaney et al.,<span class="cite-ref"><sup>[73]</sup></span> the D-ring rearrangement begins with a retro-aldol process, producing an aldehyde at the 16-position and a 17,20-dihydroxy-enol moiety. The 17,20-enol can then attack the 16-aldehyde from both sides, yielding two epimeric degradants, that is, cis-dihydroxy- and trans-dihydroxyhomotriamcinolone, as shown in pathway a, Scheme 4.37. When the D-ring expansion of triamcinolone is catalyzed by metal ions, the rearrangement mechanism via the transition metal ion complex (with 17-hydroxy-20-keto moiety) as illustrated in Scheme 4.36 can explain the stereo-specific formation of the cis-dihydroxyhomotriamcinolone through pathway b, Scheme 4.37.</p>
</td><td>

<p>Triamcinolone 是另一个皮质类固醇类药物，它与 prednisolone 的唯一区别在于其 16α-位多了一个羟基。前文曾提到 16,17-dihydroxy-20-keto 比 17-hydroxy-20-keto 结构更容易发生扩环重排。由于此羟基处于 20-位羰基的 β 位，triamcinolone 更容易发生扩环。<span class="cite-ref"><sup>[68]</sup></span> 此外，碱催化条件下，triamcinolone 的扩环重排反应将经历不同机理。由 Delaney 等人的研究<span class="cite-ref"><sup>[73]</sup></span>可知，此时的扩环重排开始于逆羟醛缩合反应，生成 16-位醛基、17,20-dihydroxy-enol。此烯醇可从两个方向进攻 16-位醛基，从而生成两个差向异构的降解产物：cis-dihydroxy- 和 trans-dihydroxyhomotriamcinolone，见 Scheme 4.37 途径 a。金属离子催化时，重排反应需经历金属离子络合的过渡态（17-hydroxy-20-keto 与离子螯合），见 Scheme 4.37 途径 b。此时将生成立体专一的 cis-dihydroxyhomotriamcinolone。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.37.png" alt="Scheme 4.37  " /><p class="caption"><span class="pic-ref">Scheme 4.37</span>  </p>
</div>
</td></tr>
<tr><td>

<p>The 21-hydroxyl group of the corticosteroids containing the 17,21-dihydroxyacetone side chain is frequently phosphorylated to make water-soluble corticosteroid phosphates for injectable formulations. These pro-drugs, for example, prednisolone phosphate,<span class="cite-ref"><sup>[74]</sup></span> and betamethasone phosphate,<span class="cite-ref"><sup>[75]</sup></span> also undergo D-homoannulation and it appears that the phosphate group may play a role in facilitating the rearrangement (Scheme 4.38).<span class="cite-ref"><sup>[75]</sup></span> In the case of betamethasone phosphate, three D-homoannulation degradants, that is, BSP isomers 1 to 3, were observed in significant quantities.</p>
</td><td>

<p>含有 17,21-二羟基丙酮侧链的皮质类固醇类药物往往会将其 21-位羟基酯化为磷酸酯，以提高其水溶性从而制成注射剂型。这些前药，比如 prednisolone phosphate <span class="cite-ref"><sup>[74]</sup></span> 和 betamethasone phosphate <span class="cite-ref"><sup>[75]</sup></span>，依然可以发生 D-homoannulation，且磷酸酯结构有可能使得重排反应更容易发生，见 Scheme 4.38。<span class="cite-ref"><sup>[75]</sup></span> 以 betamethasone phosphate 为例，扩环重排将生成为数不少的三个降解产物：BSP isomers <strong>1、2、3</strong>。</p>
</td></tr>
<tr><td>

<p>Vancomycin is a glycopeptide antibiotic that is typically used for the treatment of severe infection caused by Gram-positive bacteria<span class="cite-ref"><sup>[76]</sup></span> and often is the choice of last resort. Despite of its long history of clinical application since the 1950s, its exact structure and degradation mechanism was not elucidated until the 1980s.<span class="cite-ref"><sup>[77]</sup></span> The main degradation of vancomycin is caused by the initial cyclization of its asparagine residue (a deamidation process), followed by hydrolysis of the succinimide intermediate formed, to yield first the minor degradant called CDI-m. Furthermore, as a consequence of the expansion of the macrocyclic ring by a CH<sub>2</sub> unit during the first step of the degradation, the previously restricted chlorophenol moiety in the enlarged macrocyclic ring is now capable of rotation, which produces the major degradant, CDI-M (Scheme 4.39).</p>
</td><td>

<p>糖肽类抗生素 vancomycin 一般用于治疗革兰氏阳性菌引起的剧烈感染<span class="cite-ref"><sup>[76]</sup></span>，且往往是最后之选。虽然从 91 世纪五十年代开始便应用于临床，但直到八十年代才确定了它的准确结构和降解机理。<span class="cite-ref"><sup>[77]</sup></span> 其降解如下，首先天冬酰胺环化（脱酰胺基反应），随后丁二酰亚胺中间体(succinimide intermediate)水解生成次要降解产物 <strong>CDI-m</strong>。此外，经上述降解反应，环扩大了一个 CH<sub>2</sub>，这使得原本碍于空间位阻无法自由旋转的氯代苯环变得可随意旋转了，这将产生主要降解产物 <strong>CDI-M</strong>。详见 Scheme 4.39。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.39.png" alt="Scheme 4.39  " /><p class="caption"><span class="pic-ref">Scheme 4.39</span>  </p>
</div>
</td></tr>
<tr><td>

<p>This transformation is still referred to as &quot;rearrangement&quot;, despite the change in the overall formula of vancomycin (albeit slightly: an amino is replaced by a hydroxyl). The key step in the degradation of vancomycin illustrated above is the &quot;rearrangement&quot; of the asparagine residue. In protein and peptide drugs containing asparagine and/or aspartic acid residues, degradation caused by this rearrangement (which is typically referred to as deamidation) is one of the main degradation pathways of these drugs. The latter will be discussed in details in Chapter 7, Chemical Degradation of Biological Drugs.</p>
</td><td>

<p>这一转化过程仍然称作是“重排”，纵使 vancomycin 的分子式发生了改变（氨基被替换为羟基）。毕竟其降解反应的关键是天冬氨酸残基的重排反应。此重排反应，一般称作脱酰胺作用，往往是那些含有天冬酰胺或天冬氨酸残基的蛋白或多肽类药物的主要降解途径。这将在 第七章 进一步讨论。</p>
</td></tr>
<tr><td>

<h3 id="intramolecular-cannizzaro-rearrangement"><a href="#intramolecular-cannizzaro-rearrangement">4.5.7 Intramolecular Cannizzaro Rearrangement</a></h3>
</td><td>

<h3 id="分子内-cannizzaro-反应"><a href="#分子内-cannizzaro-反应">4.5.7 分子内 Cannizzaro 反应</a></h3>
</td></tr>
<tr><td>

<p>For corticosteroids containing a 1,3-dihydroxyacetone side chain on the D-ring, such as the ones we have just discussed in this chapter, degradation occurring due to this moiety is responsible for the vast majority of the degradation products of this important class of drugs. As we have demonstrated, various mechanisms, for example, dehydration, oxidation, and retro-aldol, are involved in different pathways of this degradation. One of these degradation mechanisms is the Cannizzaro rearrangement, which is responsible for further degradation of certain degradants possessing an a-keto-aldehyde functionality on the steroid D-ring.<span class="cite-ref"><sup>[78]</sup></span> For example, betamethasone enol aldehyde is a dehydration degradant of betamethasone which has two regioisomers (refer to Section 4.1.1). Both the E- and Z-enol aldehyde can be rehydrated to form two enol aldehyde hydrates.<span class="cite-ref"><sup>[75]</sup></span> The latter can undergo Cannizzaro rearrangement to produce four additional isomeric degradants of betamethasone.<span class="cite-ref"><sup>[71,75]</sup></span> This degradation pathway is summarized in Scheme 4.40.</p>
</td><td>

<p>某些皮质类固醇类药物分子，其 D 环侧链上含有 1,3-二羟基丙酮结构，比如本章曾经讨论过的那些，这类药物的大多数降解行为都与此结构有关。我们已经证明了此降解过程涉及多种机理，例如，失水反应，氧化反应，逆羟醛缩合反应。其中一种降解机理是 Cannizzaro 反应，当某降解产物的 D 环侧链上具有 α-酮醛结构时，则会发生此反应。<span class="cite-ref"><sup>[78]</sup></span> 例如，betamethasone 的失水降解产物 etamethasone enol aldehyde，它有两个位置异构体（参见 小节 4.1.1）。 E- 和 Z-enol aldehyde 都可被再次水解，生成enol aldehyde hydrate。 <span class="cite-ref"><sup>[75]</sup></span> 后者可发生 Cannizzaro 反应，产生额外的四个 betamethasone 的同分异构体。<span class="cite-ref"><sup>[71,75]</sup></span> 此降解途径总结于 Scheme 4.40。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.40.png" alt="Scheme 4.40  " /><p class="caption"><span class="pic-ref">Scheme 4.40</span>  </p>
</div>
</td></tr>
<tr><td>

<h2 id="cyclization"><a href="#cyclization">4.6 Cyclization</a></h2>
</td><td>

<h2 id="环化反应"><a href="#环化反应">4.6 环化反应</a></h2>
</td></tr>
<tr><td>

<h3 id="formation-of-diketopiperazine-dkp"><a href="#formation-of-diketopiperazine-dkp">4.6.1 Formation of Diketopiperazine (DKP)</a></h3>
</td><td>

<h3 id="形成环缩二氨酸"><a href="#形成环缩二氨酸">4.6.1 形成环缩二氨酸</a></h3>
</td></tr>
<tr><td>

<p>A fairly large number of small molecule drugs are dipeptides or dipeptide analogs. If the N-terminal of these dipeptides is not protected, the amino group can react with the carbonyl group of the C-terminal, resulting in the formation of a diketopiperazine (DKP) ring (Scheme 4.41). Dipeptide drugs containing a C-terminal proline residue are particularly susceptible to this cyclization,<span class="cite-ref"><sup>[79]</sup></span> which may be due to the fact that the proline residue predisposes the conformation of the dipeptides in a way favorable to the cyclization.</p>
</td><td>

<p>有许多小分子药物是二肽或二肽拟似物。若其 N-端未加保护，则氨基可与 C-端的羧基反应，生成环缩二氨酸(diketopiperazine, DKP)，见 Scheme 4.41。C-端为脯氨酸残基的二肽药物分子尤其容易发生此环化反应<span class="cite-ref"><sup>[79]</sup></span>，这是因为脯氨酸残基的构象更容易发生环化。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.41.png" alt="Scheme 4.41  " /><p class="caption"><span class="pic-ref">Scheme 4.41</span>  </p>
</div>
</td></tr>
<tr><td>

<p>Quite a few angiotensin-converting-enzyme (ACE) inhibitors such as lisinopril, enalapril, ramipril, perindopril, quinapril,<span class="cite-ref"><sup>[80]</sup></span> and moexipril,<span class="cite-ref"><sup>[81]</sup></span> are dipeptide analogs containing an unprotected N-terminal secondary amino group and a C-terminal proline residue or its analog (Figure 4.5). Hence, DKP cyclization is a common and significant degradation pathway for these drugs. DKP cyclization can be catalyzed by both acid and base.<span class="cite-ref"><sup>[82,83]</sup></span></p>
</td><td>

<p>少数血管紧张素I转化酶(ACE)抑制剂，例如 lisinopril、enalapril、ramipril、perindopril、quinapril <span class="cite-ref"><sup>[80]</sup></span> 和 moexipril <span class="cite-ref"><sup>[81]</sup></span>。它们都是二肽拟似物，且其 N-端皆未保护、C-端为脯氨酸残基或其类似物(Figure 4.5)。于是，DKP 环化是这些药物的常见且显著的降解途径。此过程可由酸或碱催化。<span class="cite-ref"><sup>[82,83]</sup></span></p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/F4.5.png" alt="Figure 4.5   可发生 DKP 环化的 ACE 抑制剂的结构式，虚线圈出了环化涉及的基团。 Structures of exemplary ACE inhibitors that undergo DKP cyclization. The groups involved in cyclization are highlighted in dotted circles." /><p class="caption"><span class="pic-ref">Figure 4.5</span>   可发生 DKP 环化的 ACE 抑制剂的结构式，虚线圈出了环化涉及的基团。<br /> Structures of exemplary ACE inhibitors that undergo DKP cyclization. The groups involved in cyclization are highlighted in dotted circles.</p>
</div>
</td></tr>
<tr><td>

<h3 id="other-cyclization-reactions"><a href="#other-cyclization-reactions">4.6.2 Other Cyclization Reactions</a></h3>
</td><td>

<h3 id="其他环化反应"><a href="#其他环化反应">4.6.2 其他环化反应</a></h3>
</td></tr>
<tr><td>

<p>Denagliptin, a dipeptidyl peptidase-4 (DPP-4) inhibitor that was being developed for type 2 diabetes, contains a primary amino and a cyano group. During pharmaceutical development regarding the stability of the drug substance and its experimental formulations, it was found that the amino group attacks the cyano group of the same molecule to produce a cyclized amidine (Scheme 4.42).<span class="cite-ref"><sup>[84]</sup></span></p>
</td><td>

<p>二肽基肽酶IV(dipeptidyl peptidase-4, DPP-4)抑制剂 denagliptin 用于治疗 2 型糖尿病。其分子结构中含有伯胺和氰基。在原料药的稳定性研究和制剂实验中发现，氨基可进攻分子内的氰基生成环脒，见 Scheme 4.42。<span class="cite-ref"><sup>[84]</sup></span></p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.42.png" alt="Scheme 4.42  " /><p class="caption"><span class="pic-ref">Scheme 4.42</span>  </p>
</div>
</td></tr>
<tr><td>

<p>The initial (3S,7S,8aS)-amidine degradant formed undergoes epimerization at the 8a-position, probably via an imine-enamine tautomerization, producing the second amidine degradant (the 3S,7S,8aR-isomer). The latter can be hydrolyzed to yield the DKP degradant.</p>
</td><td>

<p>初始降解产物 (3S,7S,8aS)-amidine 的 8a-位差向异构化生成次级降解产物 3S,7S,8aR-isomer（可能是经历了亚胺-烯胺互变异构化）。后者水解即得 DKP 降解产物。</p>
</td></tr>
<tr><td>

<h2 id="dimerizationoligomerization"><a href="#dimerizationoligomerization">4.7 Dimerization/Oligomerization</a></h2>
</td><td>

<h2 id="二聚齐聚"><a href="#二聚齐聚">4.7 二聚/齐聚</a></h2>
</td></tr>
<tr><td>

<p>In this section, degradation via dimerization is broadly defined as the chemical association of two molecules of a drug substance, which can include several types such as M + M = 2M, M + M = 2M-m, and M + X + M = M-X-M, whereby M is the molecular formula of the drug substance, m is a fragment eliminated during the dimerization, and X is a linker that typically originates from an excipient or its impurities.</p>
</td><td>

<p>在本节中，二聚降解被广泛定义为两分子的药物分子发生化学结合，它包括多种类型，例如 M + M = 2M，M + M = 2M-m 和 M + X + M = MXM，其中 M 代表药物分子，m则为二聚过程中离去的片段，而 X 是交联剂，通常来自于赋形剂及其杂质。</p>
</td></tr>
<tr><td>

<p>Drug degradation via dimerization or oligomerization can proceed by many different mechanisms. Such degradation is more likely in liquid-formulated drug products with high dose concentrations. A number of cases discussed in previous chapters and/or sections involve dimeric or oligomeric degradation: for example, the morphine dimer (pseudomorphine) formed by oxidative coupling of the drug substance (Section 3.59), the ziprasidone dimer produced by oxidation and subsequent aldol condensation (Section 4.4.1), and the captopril dimer formed by oxidative coupling of the two thiol groups (Section 3.5.6). Likewise, peptide/protein drugs containing cysteine residues can dimerize via the same oxidative coupling pathway like that of captopril to form dimers and oligomers. This topic will be discussed in Chapter 7.</p>
</td><td>

<p>药物的二聚或齐聚降解有多种机理。这种降解是更容易在高浓度液体制剂中出现。一些情况下，前文曾出现过一些二聚或齐聚降解：例如，药物分子氧化偶联生成吗啡的二聚体(pseudomorphine)，见小节 3.59；ziprasidone 受氧化，随后的羟醛缩合生成二聚体，见小节 4.4.1；以及 captopril 经硫醇基氧化偶联形成二聚体，见小节 3.5.6。类似地，含有半胱氨酸残基的多肽/蛋白类药物可以发生上述氧化偶联反应，形成二聚体和齐聚物。这将在第七章详细讨论。</p>
</td></tr>
<tr><td>

<p>Losartan potassium is a potent, first-in-class angiotensin II receptor antagonist used clinically for the treatment of hypertension. It contains a tetrazole ring, which is an isostere of the carboxylic group. The negative charge on the tetrazole ring is nucleophilic and can localize on each one of the five atoms of the ring. As a result, the tetrazole ring can attack the hydroxymethyl group attached to the imidazole ring of another losartan molecule via nucleophilic substitution, resulting in the formation of two main, dimeric degradants. It is apparent that the hydroxyl group should leave as water which is facilitated under acidic conditions (Scheme 4.43).<span class="cite-ref"><sup>[85,86]</sup></span></p>
</td><td>

<p>Losartan potassium 是首创的血管紧张素 II 受体拮抗剂，其药力强劲，临床上用于治疗高血压。它含有一个四唑环，它是羧基的电子等排物。四唑环上带有负电荷，当此负电荷处于五元环的某个原子上时则具有亲核性。其结果是，四唑环可亲核攻击另一个 losartan 分子的咪唑环上的羟甲基，从而发生二聚，主要生成两个降解产物。显然，羟基在酸性条件下很容易以水分子的形式脱去，见 Scheme 4.43。<span class="cite-ref"><sup>[85,86]</sup></span></p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.43.png" alt="Scheme 4.43  " /><p class="caption"><span class="pic-ref">Scheme 4.43</span>  </p>
</div>
</td></tr>
<tr><td>

<p>In all β-lactam antibiotics such as those in the penicillin, cephalosporin, and carbapenem families, the lactam bond is in a constrained four-membered ring and thus prone to nucleophilic attack. If the attacking nucleophile is water, hydrolytic degradants are formed, discussed in Chapter 2, Hydrolytic Degradation. If the attacking nucleophile is an amino group from another molecule of the antibiotic, which is referred to as intermolecular aminolysis, dimeric and oligomeric degradants can be produced, in particular in formulations with high concentrations of antibiotics. For example, ampicillin and amoxicillin are two antibiotics in the penicillin family, both of which contain a primary amino group. This amino group can attack the four-membered lactam ring of another molecule, resulting in the formation of dimers and oligomers (Scheme 4.44).<span class="cite-ref"><sup>[87-89]</sup></span> Ertapenem, a synthetic broad-spectrum β-lactam antibiotic in the carbapenem family, contains a secondary amino group and hence, it also degrades via aminolytic dimerization. Once the four-membered lactam ring is opened during the aminolysis, the enamine moiety, unmasked from the previously fused five-membered ring, can tautomerize to produce the corresponding imine, resulting in two interchangeable dimeric degradants (dimers I and II in Scheme 4.45).<span class="cite-ref"><sup>[90]</sup></span> Furthermore, the benzoic acid moiety of ertapenem was also found to be capable of opening the lactam ring, producing an anhydride intermediate which rearranges via acyl migration to ertapenem dimer III with a more stable benzoyl amide linkage. Finally, another two dimeric degradants (dimer-H<sub>2</sub>O a and dimer-H<sub>2</sub>O b) can also be formed through intermolecular amide linkages between the secondary amino group and the two carboxylic groups from another ertapenem molecule.</p>
</td><td>

<p>所有的β-内酰胺类抗生素如青霉素类，头孢类和碳青霉烯类，其内酰胺键皆处于张力较大的四元环上，很容易发生亲核攻击。若攻击亲核试剂为水，则发生水解降解，这已经在第二章中讨论过。若亲核试剂是另一个抗生素分子的氨基，则称为分子间的氨解。此时会生成二聚和齐聚降解产物，特别是在高浓度的制剂产品中尤其显著。例如，青霉素类的 ampicillin 和 amoxicillin，其分子中都存在伯氨基。此氨基可以攻击另一个抗生素分子的四元内酰胺环，形成二聚体和齐聚体，见 Scheme 4.44。<span class="cite-ref"><sup>[87-89]</sup></span> Ertapenem 是一种人工合成的广谱 β-内酰胺类抗生素，属于碳青霉烯类，其分子中含有仲氨基，故而，它也会发生类似的二聚降解。氨解时，四元内酰胺环一旦打开，原本禁锢在五元环中的烯胺结构可互变异构为相应的亚胺，这将产生两个可互相转化的二聚体降解产物（dimer I 和 II），见 Scheme 4.45。<span class="cite-ref"><sup>[90]</sup></span> 此外，还发现 ertapenem 的苯甲酸基团也能进攻内酰胺环使之开环，发生重排反应：酰基迁移形成更稳定的苯甲酰胺，生成 ertapenem dimer III。最后，另外两个二聚体降解产物（dimer-H<sub>2</sub>O a 和 dimer-H<sub>2</sub>O b），也是仲氨基和另一分子 ertapenem 的羧基形成“分子间”酰胺键而生成的。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.45.png" alt="Scheme 4.45  " /><p class="caption"><span class="pic-ref">Scheme 4.45</span>  </p>
</div>
</td></tr>
<tr><td>

<p>Biapenem, another antibiotic in the carbapenem family, does not contain an amino group unlike the antibiotics discussed above. Hence, it cannot undergo a direct dimerization described above. Nevertheless, the carboxyl group on the 4-membered lactam ring is capable of attacking the lactam ring of another biapenem molecule, under acidic or basic conditions and elevated temperature, to produce a dimeric anhydride intermediate. The latter can rearrange via acyl migration to dimer A.<span class="cite-ref"><sup>[91]</sup></span> Up to this point, this degradation pathway is analogous to pathway b in the degradation of ertapenem (Scheme 4.45) in terms of the reaction between a carboxyl and the lactam ring. The carboxyl group on the five membered ring in dimer A can now attack the remaining lactam ring triggering another sequence of ring opening followed by acyl migration, which ultimately yields isomeric dimer B containing a fused DKP ring. Separately, hydrolysis of the lactam ring of biapenem gives rise to another set of isomeric degradants, Impurity I and II. All the above degradation pathways of biapenem are summarized in Scheme 4.46. The nomenclature of the degradants of biapenem is the same as in the original publication.<span class="cite-ref"><sup>[91]</sup></span></p>
</td><td>

<p>Biapenem 也是碳青霉烯类抗生素，不同的是，它不再含这样的氨基。因此，它不会发生上述的直接二聚反应。但是，在酸性、碱性或升温条件下，四元内酰胺环上的羧基可攻击另一个 Biapenem 分子的内酰胺环，二聚成为酸酐。此酸酐中间体可发生酰基迁移重排为 dimer A。<span class="cite-ref"><sup>[91]</sup></span> 至此，该降解途径类似于 ertapenem 的羧基和内酰胺环之间的反应（Scheme 4.45 途径 b）。Dimer A 的五元环上的羧基，可攻击残存的内酰胺环，触发再一次的酰基迁移，最终产生异构体 dimer B，它含有一个稠合的 DKP 环。此外，biapenem 的内酰胺环水解可产生另外一对降解产物：Impurity I 和 II，都是 dimer A 和 B 的同分异构体。整个降解反应总结于 Scheme 4.46。Biapenem 降解产物的命名与原论文保持一致。<span class="cite-ref"><sup>[91]</sup></span></p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.46.png" alt="Scheme 4.46  " /><p class="caption"><span class="pic-ref">Scheme 4.46</span>  </p>
</div>
</td></tr>
<tr><td>

<p>Imipenem, the first antibiotic in the carbapenem family which was discussed in Chapter 2, Hydrolytic Degradation, also degrades to form a similar DKP dimer via the same mechanism as shown above.<span class="cite-ref"><sup>[92]</sup></span></p>
</td><td>

<p>Imipenem 是第一个碳青霉烯类抗生素，曾在第二章中提到。它也能经由上述机理生成相应的 DKP 二聚体。<span class="cite-ref"><sup>[92]</sup></span></p>
</td></tr>
<tr><td>

<p>The above cases of degradation via dimerization, either belong to type M + M = 2M (like ampicillin, amoxicillin, ertapenem, biapenem, and imipenem) or type M + M = 2M - m (like losartan). Sometimes, a dimeric degradant can be formed via a linker X as in type M + X + M = M-X-M, and formaldehyde frequently plays such a linker role.</p>
</td><td>

<p>上述的二聚化降解，有些属于 M + M = 2M 型（如 ampicillin、amoxicillin、ertapenem、biapenem、imipenem），有些则属于 M + M = 2M - m（如 losartan ）。但有些时候，则发生 M + X + M = MXM 型二聚降解，两分子 API 通过交联剂 X 反应形成二聚体，而甲醛是常见的交联剂。</p>
</td></tr>
<tr><td>

<p>Formaldehyde is a synthon employed in the synthesis of certain drug substances. For example, hydrochlorothiazide, a common diuretic drug, is made by the reaction of 5-chloro-2,4-disulfamylaniline with formaldehyde. Nevertheless, formaldehyde can also cause undesirable dimerization of hydrochlorothiazide during this synthesis. Additionally, it is known that hydrochlorothiazide decomposes via a retro-synthetic pathway to regenerate formaldehyde. Hence, the hydrochlorothiazide dimer may be a degradant as well. Two studies of this impurity were published independently by Franolic et al.<span class="cite-ref"><sup>[93]</sup></span> and Fang et al.<span class="cite-ref"><sup>[94]</sup></span> at approximately the same time. The structure of this impurity was unequivocally identified by the use of 2D NMR spectroscopy.<span class="cite-ref"><sup>[94]</sup></span> The process and degradation chemistry of hydrochlorothiazide discussed above is summarized in Scheme 4.47.</p>
</td><td>

<p>在某些药物的合成中，会使用甲醛为合成子。例如，常用的利尿剂 hydrochlorothiazide ，就是由 5-chloro-2,4-disulfamylaniline 和甲醛反应制备的。然而在此步合成中，甲醛会导致副产物 hydrochlorothiazide 二聚体的生成。此外，人们发现 hydrochlorothiazide 降解时会发生逆反应再次释放出甲醛。因此，二聚体既是副产物也是降解产物。Franolic <span class="cite-ref"><sup>[93]</sup></span> 和 Fang 等人<span class="cite-ref"><sup>[94]</sup></span> 都对此杂质进行了研究，并机会同时发布了研究成果。借助二维 NMR 谱精确确定了该杂质的结构。<span class="cite-ref"><sup>[94]</sup></span> Hydrochlorothiazide 的降解过程及相关反应见 Scheme 4.47。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.47.png" alt="Scheme 4.47  " /><p class="caption"><span class="pic-ref">Scheme 4.47</span>  </p>
</div>
</td></tr>
<tr><td>

<p>In the above pathway, the key steps are the sequential formation of two electrophiles, that is, an imine and iminium cation, followed by respective nucleophilic attacks. In the first key step, the imine intermediate is attacked by the intramolecular sulfonylamido group. In the second key step, the hydrochlorothiazide formed condenses with another molecule of formaldehyde to produce the iminium intermediate. Attack on the latter by the unsubstituted sulfonylamido group of hydrochlorothiazide yields the dimer.</p>
</td><td>

<p>在上述途径中，反应的关键是顺次形成两个亲电试剂，即亚胺和亚胺离子，进而发生相应的亲核攻击。在第一步反应中，分子内的磺酰胺基团攻击亚胺中间体。在第二步中，hydrochlorothiazide 与另一分子的甲醛缩合产生中间体亚胺离子。另一分子 Hydrochlorothiazide 的游离的磺酰胺基团攻击亚胺离子，最终生成二聚体。</p>
</td></tr>
<tr><td>

<p>Formaldehyde is also an impurity present in a number of pharmaceutical excipients, like PEG and polysorbate, which are prone to autooxidation during which process various oxidative degradants, including formaldehyde, can be produced.<span class="cite-ref"><sup>[95]</sup></span> Hence, in drug products formulated with these excipients, degradation of the APIs via formaldehyde/methylene-bridged dimerization may be possible. For example, a stability study of an experimental formulation of O<sup>6</sup>-benzylguanine (NSC-637037), a drug candidate that demonstrated anti-cancer potential, in aqueous PEG 400 showed that a methylene-bridged dimer is the main degradant.<span class="cite-ref"><sup>[96]</sup></span> The degradation is most likely to proceed through the imine intermediate, followed by nucleophilic attack by a second molecule of O<sup>6</sup>-benzylguanine as illustrated in Scheme 4.48.</p>
</td><td>

<p>甲醛也是一些药物辅料中存在的杂质。<span class="cite-ref"><sup>[95]</sup></span> 例如 PEG 和聚山梨醇酯都容易发生自然氧化，在此过程中将产生多种氧化降解产物，其中就含有甲醛。因此，使用这些赋形剂的制剂产品中，可能出现两分子 API 通过甲醛或亚甲基键连的二聚降解产物。例如，备选药物 O<sup>6</sup>-benzylguanine (NSC-637037) 已被证明具有抗癌功效，在某实验性制剂产品的稳定性研究中发现，在 PEG 400 的水溶液中，显示产生了亚甲基桥联的二聚体，而且是主要降解物。<span class="cite-ref"><sup>[96]</sup></span> 此降解最有可能是生成了亚胺中间体，然后另一分子的 O<sup>6</sup>-benzylguanine 亲核攻击此亚胺中间体。如 Scheme 4.48 所示。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.48.png" alt="Scheme 4.48  " /><p class="caption"><span class="pic-ref">Scheme 4.48</span>  </p>
</div>
</td></tr>
<tr><td>

<p>In drug entities containing activated aromatic rings that can react with formaldehyde via electrophilic aromatic substitution, formaldehyde-bridged dimerization is also possible. For instance, an indolocarbazole derivative (Scheme 4.49) based on rebeccamycin, a potent inhibitor of topoisomerase I in a phase III clinical trial for anti-cancer treatment, contains two fused 6-hydroxyindole moieties. In experimental formulations containing high concentrations of the drug candidate in aqueous PEG, two dimeric degradants formed via a methylene linkage between the respective 5-hydroxyindole moieties were observed and fully characterized by liquid chromatography-mass spectroscopy (LC-MS), LC-MS/MS, and NMR analysis (1D and 2D).<span class="cite-ref"><sup>[97]</sup></span> The formation of the two dimeric degradants should proceed through the Lederer-Manasse hydroxylalkylation mechanism, which is shown in Scheme 4.49.</p>
</td><td>

<p>某些药物分子中含有活化的芳香环，则甲醛可进攻芳环发生亲电取代反应，因此有可能生成由“甲醛”键连的二聚体。例如，基于 rebeccamycin 改造而成的 indolocarbazole 衍生物（Scheme 4.49），是抗癌功效的拓扑异构酶 I 抑制剂，现已进入 III 期临床试验，其分子中含有两个稠合的 6-羟基吲哚环。其试验性制剂是含有高浓度 API 的 PEG 水溶液，研究中观测到了它的两个二聚降解产物（亚甲基连接两个 API 分子的 6-羟基吲哚环），并借助 LC-MS、LC-MS/MS 和 NMR (1D and 2D) 进行了详细表征。<span class="cite-ref"><sup>[97]</sup></span> 这两个二聚体降解产物的形成乃是经历了 Lederer-Manasse 羟烷基化机理，如 Scheme 4.49 所示。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.49.png" alt="Scheme 4.49  " /><p class="caption"><span class="pic-ref">Scheme 4.49</span>  </p>
</div>
</td></tr>
<tr><td>

<h2 id="miscellaneous-degradation-mechanisms"><a href="#miscellaneous-degradation-mechanisms">4.8 Miscellaneous Degradation Mechanisms</a></h2>
</td><td>

<h2 id="其他降解机理"><a href="#其他降解机理">4.8 其他降解机理</a></h2>
</td></tr>
<tr><td>

<h3 id="diels-alder-reaction"><a href="#diels-alder-reaction">4.8.1 Diels-Alder Reaction</a></h3>
</td><td>

<h3 id="diels-alder-反应"><a href="#diels-alder-反应">4.8.1 Diels-Alder 反应</a></h3>
</td></tr>
<tr><td>

<p>Although the Diels-Alder reaction is widely utilized in synthetic organic chemistry, drug degradation via Diels-Alder does not appear to be common.</p>
</td><td>

<p>虽然在有机合成中 Diels-Alder 反应应用广泛，但在药物分子的降解中并不怎么常见。</p>
</td></tr>
<tr><td>

<p>One rare case is ethacrynic acid, a diuretic drug whose degradation behavior is mostly attributable to its α,β-unsaturated carbonyl functionality, discussed in Section 4.3. The same functionality makes ethacrynic acid susceptible to degradation via the 4 + 2 Diels-Alder addition mechanism (Scheme 4.50),<span class="cite-ref"><sup>[98]</sup></span> which gives another dimeric degradant that is different from the one mentioned in Section 4.3.</p>
</td><td>

<p>有一个罕见的例子，利尿剂 ethacrynic acid，结构中含有 α,β-不饱和羰基，其降解行为在 小节 4.3 已然讨论过。此结构使得 ethacrynic acid 还容易发生 4 + 2 Diels-Alder 加成，见 Scheme 4.50。<span class="cite-ref"><sup>[98]</sup></span> 而这将生成另一种二聚体降解产物。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.50.png" alt="Scheme 4.50  " /><p class="caption"><span class="pic-ref">Scheme 4.50</span>  </p>
</div>
</td></tr>
<tr><td>

<h3 id="degradation-via-reduction-or-disproportionate"><a href="#degradation-via-reduction-or-disproportionate">4.8.2 Degradation via Reduction or Disproportionate</a></h3>
</td><td>

<h3 id="还原或歧化反应造成的降解"><a href="#还原或歧化反应造成的降解">4.8.2 还原或歧化反应造成的降解</a></h3>
</td></tr>
<tr><td>

<p>As we discussed quite extensively in Chapter 3, Oxidative Degradation, drug degradation via various oxidative pathways is one of the two most frequently observed events in drug stability (or instability; the other being hydrolytic degradation). This is understandable since in the vast majority cases, the ultimate oxidizing agent is molecular oxygen. However, drug degradation via reduction is rare, if not impossible, owing to the lack of reducing agents that are capable of reductively degrading the drug substances. Hence the current author was quite surprised to come across a paper that reported the reductive degradation of rabeprazole sodium in a simulated intestinal fluid (a pH 6.8, 50 mM phosphate buffer).<span class="cite-ref"><sup>[99]</sup></span> Upon careful review of the experimental evidence presented, it appears that the &quot;reductive degradation&quot; of rabeprazole, a proton-pump inhibitor containing a sulfoxide moiety, may be a case of degradation via disproportionation. In such a case, when one molecule of rabeprazole is reduced to the reductive degradant, rabeprazole thioether, as observed by the original authors, another molecule of rabeprazole is most likely to be oxidized to form rabeprazole sulfone. In the chromatograms presented in the paper, it can be seen that when the late-eluting peak corresponding to rabeprazole thioether started to occur and increased over time, two early eluting peaks (one major and one minor; both immediately after the void volume), occurred and also increased in their peak areas. The sum of the peak areas apparently increased in sync with the thioether peak area. It seems that these peaks could be rabeprazole sulfone and a related oxidative degradant, which might be overlooked during the original study because they eluted immediately after the void volume in the high-performance liquid chromatography (HPLC) analysis.</p>
</td><td>

<p>正如在第3章广泛讨论的那样，药物分子通过各种氧化途径发生的氧化降解是药稳定性研究中最常观察到的两种降解方式之一（另一个是水解降解）。这是可以理解的，因为在绝大多数的情况下，氧气是最基本的氧化剂。然而，通常情况下，经还原反应发生药物降解则是罕见的，毕竟体系中一般缺少还原剂来完成还原降解。因此，笔者巧遇此篇论文时非常惊讶：在模拟肠液(pH = 6.8，50 mM 磷酸盐缓冲液)中，rabeprazole sodium 发生了还原降解。<span class="cite-ref"><sup>[99]</sup></span> rabeprazole 是含亚砜基团的质子泵抑制剂 ，实验结果证明产生了还原降解产物，这可能会出现了歧化反应。在此例中，原作者观测到，一个分子的 rabeprazole 的被还原为 rabeprazole thioether，另一分子的 rabeprazole 很有可能是被氧化成了rabeprazole sulfone。论文中给出了相应的色谱图，后洗脱的峰会随着时间的推移不断增加，它对应于 rabeprazole thioether，此峰出现时，还出现了两个较早被洗脱的峰（紧随死体积之后，一大一小两个峰），且会随着前者增加，这两个峰的面积也相应增加。且这两个峰的总面积明显地会与硫醚的峰面积同步增长。由此看来，这两个峰可能就是 rabeprazole sulfone 和另一个相关的氧化降解产物，在原作者的研究它们很可能被忽视了，毕竟它们的保留时间极小，紧随着高效液相色谱(HPLC)的死时间。</p>
</td></tr>
<tr><td colspan=2>

<div class="figure">
<img src="png/S4.51.png" alt="Scheme 4.51  " /><p class="caption"><span class="pic-ref">Scheme 4.51</span>  </p>
</div>
</td></tr>
<tr><td>

<p>Furthermore, since the thioether degradant elutes much later than rabeprazole, which is a sulfoxide, it is quite possible that the sulfone degradant is more polar than rabeprazole and hence elutes very closely to the void volume under the HPLC conditions employed. The likely scenario of rabeprazole degradation via disproportionation is shown in Scheme 4.51.</p>
</td><td>

<p>此外，rabeprazole 是一个亚砜，相应的硫醚降解产物的保留时间远远大于 rabeprazole。而相应的硫砜极性更大，因此在 HPLC 分析中，rabeprazole sulfone 完全有可能紧随死体积被洗脱下来。rabeprazole 的歧化降解可能如 Scheme 4.51 所示。</p>
</td></tr>
</table>

<h2 id="references"><a href="#references">References</a></h2>
<ol style="list-style-type: decimal">
<li><p>V. R. Mattox, J. Am. Chem. Soc., 1952, 74, 4340.</p></li>
<li><p>M. L. Lewbart and V. R. Mattox, J. Org. Chem., 1964, 29, 513.</p></li>
<li><p>T. Hidaka, S. Huruumi, S. Tamaki, M. Shiraishi and H. Minato, Yakugaku Zasshi, 1980, 100, 72.</p></li>
<li><p>M. Li, B. Chen, M. Lin, T.-M. Chan, X. Fu and A. Rustum, Tetrahedron Lett, 2007, 48, 3901.</p></li>
<li><p>B. Chen, M. Li, M. Lin, G. Tumambac and A. Rustum, Steroids, 2009,74, 30.</p></li>
<li><p>M. Li, B. Chen, M. Lin and A. Rustum, Am. Pharm. Rev., 2008, 1, 98.</p></li>
<li><p>M. Shou, W. A. Galinada, Y.-C. Wei, Q. Tang, R. J. Markovich and A. M. Rustum, J. Pharm. Biomed. Anal., 2009, 50, 356.</p></li>
<li><p>P. Kaur, G. Wilmer, Y.-C. Wei and A. M. Rustum, Chromatographia, 2010, 71, 805.</p></li>
<li><p>R. M. Bianchini, P. M. Castellano and T. S. Kaufman, J. Pharm. Biomed. Anal., 2008, 48, 1151.</p></li>
<li><p>P. J. Atkins, T. O. Herbert and N. B. Jones, Int. J. Pharm., 1986, 30, 199.</p></li>
<li><p>T. Cachet, G. Van den Mooter, R. Hauchecorne, C. Vinckier and J. Hoogma.rt.ens, Int. J. Pharm., 1989, 55, 59.</p></li>
<li><p>P. Alam, P. C. Buxton, J. A. Parkinson and J. Barber, J. Chem. Soc. Perkin Trans. 2, 1995, 1163.</p></li>
<li><p>A. Hassanzadeh, M. Helliwellb and J. Barber, Org. Biomol. Chem., 2006, 4, 1014.</p></li>
<li><p>J. Barber, J. I. Gyi, L. Lian, G. A. Morris, D. A. Pye and J. K. Sutherland, J. Chem. Soc. Perkin Trans., 1991, 2, 1489.</p></li>
<li><p>E. F. Fiese and S. H. Steffen, J. Antimicrob. Chemother., 1990, 25(Suppl. A), 39.</p></li>
<li><p>X. W. Teng, D. C. Cutler and N. M. Davies, Int. J. Pharm., 2003, 259, 129.</p></li>
<li><p>S. Sahasranaman, M. Issar, G. Toth, G. Horvath and G. Hochhaus, Pharmazie, 2004, 59, 367.</p></li>
<li><p>I. Nikcevic, P. Sajonz, M. Li, R. Markovich, A. Rustum, Challenges in the Analytical Method Development for Drug Product Containing a Steroid Active Pharmaceutical Ingredient, presentation at Pittcon 2011, Session number 600-4.</p></li>
<li><p>J. B. Stenlake, R. D. Waigh and G. H. Dewar, Eur J. Med. Chem., 1981,16, 515.</p></li>
<li><p>R. M. Welch, A. Brown, J. Ravitch and R. Dahl, Clin. Pharmacol. Ther., 1995, 58, 132.</p></li>
<li><p>G. R. Humphrey, R. A. Miller, P. J. Pye, K. Rossen, R. A. Reamer, A. Maliakal, S. S. Ceglia, E. J. J. Grabowski, R. P. Volante and P. J. Reider, J. Am. Chem. Soc., 1999, 121, 11261.</p></li>
<li><p>O. Almarsson, R. A. Seburg, D. Godshall, E. W. Tsai and M. J. Kaufman, Tetrahedron, 2000, 56, 6877.</p></li>
<li><p>Z. Zhao, X.-Z. Qin and R. A. Reed, J. Pharm. Biomed. Anal., 2002, 29, 173.</p></li>
<li><p>S. G. Jivani and V. G. Stella, J. Pharm. Sci., 1985, 74, 1274.</p></li>
<li><p>R. K. Palsmeier, D. M. Radzik and C. E. Lunte, Pharm. Res., 1992, 9, 933.</p></li>
<li><p>X. Zhang and C. S. Foote, J. Am. Chem. Soc., 1993, 115, 8867.</p></li>
<li><p>M. Li, B. Conrad, R. G. Maus, S. M. Pitzenberger, R. Subramanian, X. Fang, J. A. Kinzer and H. J. Perpall, Tetrahedron Lett., 2005, 46, 3533.</p></li>
<li><p>Y. L. Lee, J. Padula and H. Lee, J. Pharm. Sci., 1988, 77, 81.</p></li>
<li><p>A. S. Kalgutkar, I. Gardner, R. S. Obach, C. L. Shaffer, E. Callegari, K. R. Henne, A. E. Mutlib, D. K. Dalvie, J. S. Lee, Y. Nakai, J. P. O0Donnell, J. Boer and S. P. Harriman, Curr. Drug Metab., 2005, 6, 161.</p></li>
<li><p>M. Burg and N. Green, Kidney Int., 1973, 4, 301.</p></li>
<li><p>D. A. Koechel, Ann. Rev. Pharmacol. Toxicol., 1981, 21, 265.</p></li>
<li><p>D. E. Duggan and R. M. Noll, Arch. Biochem. Biophys., 1965, 109, 388.</p></li>
<li><p>D. A. Koechel and E. J. Cafruny, J. Med. Chem., 1973, 16, 1147.</p></li>
<li><p>D. A. Koechel and E. J. Cafruny, J. Pharmacol. Exp. Ther., 1975, 192, 179.</p></li>
<li><p>R. J. Yarwood, W. D. Moore and J. H. Collett, J. Pharm. Sci., 1985, 74, 220.</p></li>
<li><p>A. Marin, A. Espada, P. Vidal and C. Barbas, Anal. Chem., 2005, 77, 471.</p></li>
<li><p>J. Wong, L. Wiseman, S. Al-Mamoon, T. Cooper, L.-K. Zhang and T.-M. Chan, Anal. Chem., 2006, 78, 7891.</p></li>
<li><p>L. Ding, X. Wang, Z. Yang and Y. Chen, J. Pharm. Biomed. Anal., 2008, 46, 282.</p></li>
<li><p>G. Orgovan, K. Tihanyi and B. Noszal, J. Pharm. Biomed. Anal., 2009,50, 718.</p></li>
<li><p>C.-Y. Liang, Y. Yang, M. A. Khadim, G. S. Banker and V. Kumar, J. Pharm. Sci., 1995, 84, 1141.</p></li>
<li><p>B. J. Fish and G. P. R. Carr, J. Chromatogr., 1986, 353, 39.</p></li>
<li><p>C. A. Janicki and C. Y. Ko, Anal. Profiles Drug Subst., 1980, 9, 341.</p></li>
<li><p>S. Fors, in The Maillard Reaction in Foods and Nutrition, ed. G.R. Waller and M.S. Feather, ACS Symposium Series, Vol. 215, American Chemical Society, Chapter 12, pp. 185-286.</p></li>
<li><p>M. Jun, Y. Shao, C.-T. Ho, U. Koetter and S. Lech, J. Agric. Food Chem., 2003, 51, 6340.</p></li>
<li><p>J. Hong, J. C. Shah and M. D. McGonagle, J. Pharm. Sci., 2011,100, 2703.</p></li>
<li><p>D. E. Guttman and F. D. Meister, J. Am. Pharm. Assoc. Sci. Ed., 1958, 47, 773.</p></li>
<li><p>J. Hansen and H. Bundgaard, Int. J. Pharm., 1980, 6, 307.</p></li>
<li><p>H. S. Wendler in Molecular Rearrangements, Part Two, ed. P. Mayo, Interscience, New York, 1967, pp. 1067-1075.</p></li>
<li><p>K. Florey in Analytical Profiles of Drug Substances, Vol. 12, ed. K. Florey, Academic Press, New York, 1983, p. 277-324.</p></li>
<li><p>M. Li, B. Chen, S. Monteiro and A. M. Rustum, Tetrahedron Lett., 2009, 50, 4575.</p></li>
<li><p>H.-Y. Young, C.-T. Chiang, Y.-L. Huang, F. P. Pan and G.-L. Chen, J. Food Drug Anal., 2002, 10, 149.</p></li>
<li><p>N. Fukutsu, T. Kawasaki, K. Saito and H. Nakazawa, J. Chromatogr. A, 2006, 1129, 153.</p></li>
<li><p>INVEGA®, Scientific discussion, European Medicines Agency; http:// www.ema.europa.eu/docs/en_GB/document_library/EPAR_-<em>Scientific</em> Discussion/human/000746/WC500034928.pdf.</p></li>
<li><p>C. Danel, N. Azaroual, A. Brunel, D. Lannoy, P. Odou, B. D^caudin, G. Vermeersch, J.-P. Bonte and C. Vaccher, Tetrahedron: Asymmetry, 2009, 20, 1125.</p></li>
<li><p>D. Stepensky, M. Chorny, Z. Dabour and I. Schumacher, J. Pharm. Sci., 2004, 93, 969.</p></li>
<li><p>R. J. Strife, I. Jardine and M. Colvin, J. Chromatogr., 1980, 182, 211.</p></li>
<li><p>J. H. Beijnen, J. J. M. Holthuis, H. G. Kerkdijk, O.A.G.J. van der Houwen, A. C. A. Paalman, A. Bult and W. J. M. Underberg, Int. J. Pharm., 1988, 41, 169.</p></li>
<li><p>C. Dugave and L. Demange, Chem. Rev., 2003, 103, 2475.</p></li>
<li><p>S. Zhang, J. Xing and D. Zhong, J. Pharm. Sci., 2004, 93, 1300.</p></li>
<li><p>W. H. Okamura, M. M. Midland, M. W. Hammond, N. Abd Rahman, M. C. Dormanen, I. Nemere and A. W. Norman, J. Steroid Biochem. Mol. Biol, 1995, 53, 603.</p></li>
<li><p>M. Brandl, X. Y. Wu, Y. Liu, J. Pease, M. Holper, E. Hooijmaaijer, Y. Lu and P. Wu, J. Pharm. Sci., 2003, 92, 1981.</p></li>
<li><p>N. Hashimoto and K. Hirano, J. Pharm. Sci., 1998, 87, 1091.</p></li>
<li><p>A. Ruegger, M. Kuhn, H. Lichti, H.-R Loosli, R. Huguenin, C. Quiquerez and A. Von Wartburg, Rifai. Helv. Chim. Acta, 1976, 59, 1075.</p></li>
<li><p>G. J. Friis and H. Bundgaard, Int. J. Pharm., 1992, 82, 79.</p></li>
<li><p>X. Liu, J. Zhang, L. Song, B. C. Lynn and T. G. Burke, J. Pharm. Biomed. Anal., 2004, 35, 1113.</p></li>
<li><p>L. Ruzicka and H. F. Meldahi, Helv. Chim Acta, 1938, 21, 1760.</p></li>
<li><p>D. N. Kirk and M. P. Hartshorn, Steroid Reaction Mechanisms, Elsevier, Amsterdam, 1969, pp. 294-301.</p></li>
<li><p>N. L. Wendler, in Molecular Rearrangements, ed. P. de Mayo, Interscience, New York, 1964, Vol. 2, pp. 1099-1101; pp. 1114-1121.</p></li>
<li><p>D. N. Kirk and C. R. McHugh, J. Chem. Soc. Perkin Trans 1, 1978, 1, 73.</p></li>
<li><p>D. Dekker and J. H. Beijnen, Pharm. Weekbl., Sci. Ed., 1980, 2, 112.</p></li>
<li><p>D. Dekker and J. H. Beijnen, Acta Pharm. Suec., 1981, 18, 185.</p></li>
<li><p>A. Liguori, F. Perri and C. Siciliano, Steroids, 2006, 71, 1091.</p></li>
<li><p>E. J. Delaney, R. G. Sherrill, V. Palaniswamy, T. C. Sedergran and S. P. Taylor, Steroids, 1994, 59, 196.</p></li>
<li><p>J. H. Beijnen and D. Dekker, Pharm. Weekbl., Sci. Ed., 1984, 6, 1.</p></li>
<li><p>M. Li, X. Wang, B. Chen, T.-M. Chan and A. Rustum, J. Pharm. Sci., 2009, 98, 894.</p></li>
<li><p>J. C. Rotschafer, K. Crossley, D. E. Zaske, K. Mead, R. J. Sawchuck and L. D. Solem, Antimicrob. Agents Chemother., 1982, 22, 391.</p></li>
<li><p>C. M. Harris, H. Kopecka and T. M. Harris, J. Am. Chem. Soc., 1983, 105, 6915.</p></li>
<li><p>M. L. Lewbart and V. R. Mattox, J. Org. Chem., 1963, 28, 1779.</p></li>
<li><p>S. M. Steinberg and J. L. Bada, J. Org. Chem., 1983, 48, 2295.</p></li>
<li><p>B. N. Roy, G. P. Singh, H. M. Godbole and S. P. Nehate, Indian J. Pharm. Sci, 2009, 71, 395.</p></li>
<li><p>R. G. Strickley, G. C. Visor, L.-H. Lin and L. Gu, Pharm. Res., 1989, 6, 971.</p></li>
<li><p>N. F. Sepetov, M. A. Krymsky, M. V. Ovchinnikov, Z. D. Bespalova, O. L. Isakova, M. Soueek and M. Lebl, Pept. Res., 1991, 4, 308.</p></li>
<li><p>M. Beyermann, M. Bienert, H. Niedrich, L. A. Carpino and D. Sadat-Aalace, J. Org. Chem., 1990, 55, 721.</p></li>
<li><p>B. K. Josh, B. Ramsey, B. Johnson, D. E. Patterson, J. Powers, K. L. Facchine, M. Osterhout, M. P. Leblanc, R. Bryant-Mills, R. C. B. Copley and S. L. Sides, J. Pharm. Sci., 2010, 99, 3030.</p></li>
<li><p>K. E. McCarthy, Q. Wang, E. W. Tsai, R. E. Gilbert, D. P. Ip and M. A. Brooks, J. Pharm. Biomed. Anal., 1998, 17, 671.</p></li>
<li><p>Z. Zhao, Q. Wang, E. Tsai, X. Qin and D. Ip, J. Pharm. Biomed. Anal., 1999, 20, 129.</p></li>
<li><p>H. Bundgaard, Acta Pharm. Suec., 1976, 13, 9.</p></li>
<li><p>H. Bundgaard, Acta Pharm Suec., 1977, 14, 47.</p></li>
<li><p>C. Y. Lu and C. H. Feng, J. Sep. Sci, 2007, 30, 329.</p></li>
<li><p>P. Sajonz, T. K. Natishan, Y. Wu, J. M. Williams, B. Pipik, L. DiMichele, T. Novak, S. Pitzenberger, D. Dubost and O. Almarsson, J. Liq. Chro-matogr. Relat. Technol., 2001, 24, 2999.</p></li>
<li><p>M. Xia, T.-J. Hang, F. Zhang, X.-M. Li and X.-Y. Xu, J. Pharm. Biomed. Anal., 2009, 49, 937.</p></li>
<li><p>R. W. Ratcliffe, K. J. Wildonger, L. Di Michele, A. W. Douglas, R. Hajdu, R. T. Goegelman, J. P. Springer and J. Hirshfield, J. Org. Chem., 1989,54, 653.</p></li>
<li><p>J. D. Franolic, G. J. Lehr, T. L. Barry and G. Petzinger, J. Pharm. Biomed. Anal., 2001, 26, 651.</p></li>
<li><p>X. Fang, R. T. Bibart, S. Mayr, W. Yin, P. A. Harmon, J. F. McCafferty, R. J. Tyrrell and R. A. Reed, J. Pharm. Sci., 2001, 90, 1800.</p></li>
<li><p>K. C. Waterman, W. B. Arikpo, M. B. Fergione, T. W. Graul, B. A. Johnson, B. C. MacDonald, M. C. Roy and R. J. Timpano, J. Pharm. Sci., 2008, 97, 1499.</p></li>
<li><p>D. S. Bindra, T. D. Williams and V. J. Stella, Pharm. Res., 1994, 11, 1060.</p></li>
<li><p>Y. Sato, D. Breslin, H. Kitada, W. Minagawa, T. Nomoto, X.-Z. Qin and S. B. Karki, Int. J. Pharm., 2010, 390, 128.</p></li>
<li><p>R. J. Yarwood, A. J. Phillips, N. A. Dickinson and J. H. Collett, Drug Dev. Ind. Pharm., 1983, 9, 35.</p></li>
<li><p>S. Rena, M.-J. Park, H. Sah and B.-J. Lee, Int. J. Pharm., 2008, 350, 197.</p></li>
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